I’m trying to design some classic muscle cars, but I’m not quite achieving classic muscle car levels of performance. I’m wondering what kind of chassis, suspension, and tires the vehicles would have had, and also what kind of components and tuning setup the engines would have had for their standard, performance, and semi-racing models. Also, when would a car have used a three speed automatic versus a manual transmission? I read about at least one of the higher performance vehicle options in the era coming only with an automatic transmission, and at least in game the three speed transmissions tend to have performance on par with the four speed manual transmissions in terms of acceleration, occasionally being somewhat faster or slower.
Chassis type and suspensions would really depend on the specific model. Many classic muscle cars were full frame cars with monster motors, while others were unibody pony cars with monster motors. Performance is also heavily dependent on the specific car. There were cars producing 400+hp right from the factory, while others were under-rated from the factory for insurance purposes and can be tough to determine. Automatics were generally not used in what was termed a “muscle car”, but it was not completely unheard of either. Generally speaking, however, the high performance version of a car was generally a 3 or 4 speed manual in the 60s/70s while mid-trim models often sported automatics. Base models generally were also manual transmissions. I could go on, but I won’t. heheheh
Yes, this is definitely true: I think a lot of it seems to have been thanks to the rather strong popularity of drag racing in that era, based on all that I’ve read…although some automatic cars could have also rather decent 0-60 and quarter mile times depending on the setup(and the engine). Not to mention that a good number of these cars theoretically could, manual or automatic, actually reach some pretty impressive top ends(at least for that day and age) if their individual final drive ratio setups(and tires!) allowed for it(which was probably rather rare, but certainly not quite unheard of, either.); maybe 150+ mph in a few cases(the SS454 Chevelle. 
The one thing that kinda bums me out, though, is that solid lifters aren’t available in the game just yet, which were quite popular on U.S. performance engines in the '50s and '60s, IIRC.
Automatics are indeed quite capable on a drag strip. In most cases, they will consistently outdo (with proper gearing and valve body setup of course!) a manual transmission on the strip because they shift so much more consistently than a driver/clutch can shift. A very popular automatic for the drag strip surpisingly is GM’s PowerGlide 2-speed transmission, as it only has a single shift point. Not a good option for high torque/hp engines, but for street modified they do fantastic. Most muscle cars were sold with the solid intent of seeing at least some track duty (or stop-light drag racing), but the term muscle car in itself is misleading. This is why it is so specific to the car what kind of hp/tq, tires, transmission, chassis you will find on one. Factory muscle cars were fairly well defined, but drivers had a habit of taking non-muscle cars and turning them into one. This even led to some manufacturers creating non-standard muscle cars like the Cosworth Vega, the AMC Gremlin X, and the Mustang II Cobra in response to the post oil crisis markets. These were not honest-to-goodness muscle cars, but they fit the definition of being 2+2 coupes with large V8s (~5L) under the hood.
Here is a pretty good list of what was actually considered a muscle car. The general consensus is cars fitting this category began in the early 60s with the Wildcat and GTO. It is pretty safe to say by the 1974 model year, American Muscle Cars were completely dead. There were some holdouts, but they were nerfed to the point of only being hollow shells of the glorious street machines they once were. Even the coveted Corvette was completely nerfed (its inclusion on a muscle car list is academic only as it was technically a 2-seater sports car).
There have been quite a few recreations of these engines on the forums (including a few of my own), however I cannot recall any post-Steam release. If you have a specific engine, create a thread to request it. There are a few people on here who are quite adept at making these engines! I personally am heavily in favor of GM Small/Big block motors from the 50s/60s and have a few already made. My current 1955 BRC car is being powered by a 1955 Chevy 265 small block making about 200hp @ 4600rpm. It’s a pretty solid motor and is pretty close to stock production output values. I’ve already started building my 1965 BRC entry, which is powered by a 1965 Chevy 327 small block. I also have a 1970 Chevy 400 small block and a 1970 Chevy 427 big block in my barn.
EDIT: Here are the small block Chevy motors of the 60s using a 4 inch bore.
07CobaltGirl - Small Block Chevy 4.000.zip (86.8 KB)
Thanks for the information.
Any advice on carburetor setups? Thus far I’ve been using two barrel carburetors, with single carburetors for lower tier economical engines (as economical as a V-8 can be) and three carburetor units for the more performance oriented designs. The four barrel carburetors seem to be capable of higher outputs and greater fuel economy, but the Tri-Power setup seems to be a good mix of power, economy, and responsiveness. The DCOE and mechanical fuel injection units seem to lower output somewhat relative to the more conventional carburetors and have high responsiveness, but I’m not sure what advantage they would have for a classic muscle car relative to conventional carburetors. DCOE seems similar to Quadrajet, which seems to have been more of a luxury option, while mechanical fuel injection was used due to its high responsiveness and greater ease complying with strict emissions requirements.
The quadrajet was a four barrel carb. The four barrel carb was the basically the default muscle car setup, a single two barrel was a base model setup, mostly for smaller displacement v8’s (sub 300 cubic inch/ 5 liter) at least untill the carb and cafe acts in the 70’s. in the late fifties you had some factory dual Quads which was 2x 4 barrel carburetors, Chevy did that for the 409 but by the 60’s that was mostly just racers or something that cane packed in the trunk. the tripower, triple deuce, or sixpack setup was more prevelent, but still not as common, it was mostly a trim level or dealer installed option very few engines if any came exclusively with a triple carb setup. The only muscle car with dcoe I know of was the 428 hemi that had a dual setup as a option.
To add to nialloftara’s response, in actuality, the 4bbl carbs are the most efficient. Whether mechanical or vacuum secondaries, it primarily functions as a 2bbl carb until it reaches it’s secondary actuation point. The way a 4bbl is setup, is a very small set of primaries and a larger set of secondaries. Mechanical secondaries are opened at a specific throttle point, while vacuum secondaries are generally only opened for downshifts or [near] WOT. 4bbl carbs were very rare in the 50s, but became far more common during the Muscle Car Era of the 60s. The Tri-Power, Triple-Deuce, or Six-Pack setup was a very common setup in the earlier years (50s - early 60s) for performance. They most often functioned as a “6bbl” carb and were staged as primaries, [mechanical] secondaries, and [mechanical] auxiliaries, so they would open at different points feeding much more fuel into the engine during high load conditions. This gave a tremendous boost to power output, but at a heavy cost to economy. Dual 4bbls function similarly and were generally reserved for very high performance engines, and really not used on smaller displacement engines (from the factory).
It seems most V8 engines of the era were oversquare, sometimes quite so. That seems somewhat odd if only a few engines were used for drag racing, street racing, etc., with large engines being common because of the large size of the cars. Why weren’t square or undersquare designs used if they are more for operation at lower engine speeds and produce a lot of torque? Aren’t most of the advantages of oversquare design only seen at higher engine revolutions?
Short stroke motors (oversquare) are more reliable for daily use. Long stroke motors (undersquare) make a lot of torque, but really limit RPMs. With the OHV engines of the era, higher displacement was better achieved by increasing bore. Stroking a motor takes a lot of engineering to function the same way. Boring a motor (within reasonable limits) was as simple as re-honing the cylinder walls to accommodate a larger piston. Also remember, in an OHV engine a higher revving motor would still be under 6k (under 5k was far more common) most of the time. Nearly all American engines of the era were OHV, with only a handful of exceptions. The Chevrolet SB 302 was a destroked SB 327 using a crankshaft from a SB 283. This gave it a bore of 4" and stroke of 3" for a ratio of 1.33! Not surprisingly, it was used for SCCA Trans Am racing and was quite capable of reaching 7000 RPM, while producing 350+ hp on a dyno. Not bad for a street engine measuring less than 5.0L. Keep in mind, this was being used in a car weighing less than 3500 pounds.
Short-version: There are advantages and disadvantages to every engine configuration you can come up with.
Some larger engines are unable to fit their racing intakes inside of the engine bay. Is there anything that can be done to clear an opening for them in game?
You mean the engine fits but the stacks are sticking through the hood model? If that’s the case, you can add a hood buldge from the bonnet tab to cover them up, or my personal favorite,cut a hole in your hood with one of the non mesh covered grills and show them off.
http://www.automationgame.com/phpBB3/download/file.php?id=8646
That’s how I do it. Bulging hoodlines to cover bulging V8s. 
imagizer.imageshack.us/v2/1280x1024q90/913/Sad9o4.png
I’ve incorporated it into most of my cars, so you just never know what is under the hood of an SME.
I meant more in terms of engines that are too tall for the engine bay height, as opposed to merely clipping through.
Ahh have you played around with different carb setups? I thing the triple double setup is the lowest height with a normal intake.
Although already posted on the engine sharing forum, here’s my recreation of several variants in the Ford FE series (1958-76). Used in many vehicles of the era, including the Mustang, GT-40 and AC Cobra. The carb set-ups mentioned above are reflected in the range. I hope it’s of use to you. 
WMC - FE Series.zip (105 KB)