I started my ‘car company’ a while ago and decided to start building two series of engines (base/entry versions that could be modified to fit different car models that my company choose to build). I’ve decided to update them, or make a new version every 5 years and I started building the first series of 6 litre V8 in 1962 and a 5 litre V8 in 1965. Calling them for LV67BB and LV56SB. When should I stop using carburettors (following the trend of US V8’s) and when did they stop using pushrod?

Fuel injection started to become common in America in the late 70’s and took off in the 1980’s. I would suggest switching to single-point EFI when it is unlocked (1988) as Mechanical Injection is not worth the extra cost. OHC engines generally started to appear in the 1990’s but there isn’t really a point where everyone switched, Chevy and Chrysler still use pushrods in some of their engines and while they weren’t common, OHC engines existed in America as far back as the 20’s.

Unfortunately my interest in (and therefore specific knowledge of) American cars is limited to Chevrolet Corvettes and their small-block V8s, which I am an avid fan of.

Last carburetted Corvette: 1981, although there’s a long history of optional mechanical fuel injection on Chevy V8s.
Last pushrod Corvette: Never. Because a pushrod V8 is so compact, the total valve area can easily be compensated by simply increasing displacement. The end result is lower specific power output (less kW/liter) but a much denser powerplant (more kW/m^3).

For me I’d kinda worked this system out using the games emission score.
starting in 73 a maximum 3000 emissions. this will decrease to 1300 in 77, 900 in 83, 700 in 87, 600 in 91, 400 in 97 and end at a maximum of 200 in 01. That said the last carbed v8 was the Oldsmobile 305 rocket in 1990.

For pushrods

With mechanical injection my reliability went up from 38,000 to over 50,000 km so that alone makes it worth it.
Theoretically those 10-12 points could also be “invested” in making a longer-stroke, higher revving engine. It is expensive though.

American V8s are still pushrod other than the northstar and ford, in the next update, a modern OHV setup will be added to keep OHV usefull.
Efi appeared in the late 70s and was phased out by the mid 80s.