I recently tried to build a 1970 muscle car engine and I kept getting a responsiveness of 19 or below. To me this seems dreadful for a muscle car engine and I am wondering if this is average or if I am doing something wrong.
If it is an american muscle car from the 70’s, I would say that 1 is good 
Anyway 19 should be good enough, since you’re probably using carbs which are slower that the car itself 
from a historical standpoint at 1970’s they really didn’t have the know how, well they did, but preferred to make things " Bulky" so if you can look at their responsiveness this way, They create at that time around 200 hp or less, from a v8, and tons of torque to the transmission, so you off that bat at 700-800 idle rpm have at your disposal 80 horses, with 90 torque ( give or take as its theory. ) so your now begining to haul that you push your foot down bam 1300 but your now generating your torque that gets you moving 120 torque, so its moving you and your prolly going to be moving a hunk of junk thats two tons, so your going about 20 mph at this point, now inertia kicks in, you gun the pedal, tires squeal you now have access to your 230 torque as it revs slowly but meaningfully as you have to imagine the piston heads at this point in our time weight a good 8 lbs or MORE! so you have all that weight coming down and generating tons of torque, so in theory, your low responsiveness is replaced with the fact your engine is going to push some serious torque to your wheels.(this is why a automatic transmission is better than a standard for drag competitions, it directly converts your torque with a " torque " convert 
) good day 
Conversions for LBS to KG 2.2xlbs = KG ( very estimated.)
A torque converter INCREASES the torque when the engine is revving higher than what the gearbox is. Thats all.
Also, automatics are more popular in drag racing as with a good shift kit they can shift faster than you or I can blink.
The 1970’s were a bad time for muscle cars. Emissions standards and CAFE standards came down on the muscle car hard. They started to measure output in SAE versus Gross, meaning the output of the engine must be measured using a non-cherry picked unit in full dress as it would leave the factory in a car (this means today’s mustang 5.0 making 412HP SAE would produce closer to 550HP Gross). Compression ratios lowered, carburetors shrank, cam shafts were less aggressive in the pursuit of fuel economy and lower emissions. All these factors added to the loss of HP and responsiveness.
An automatic transmission is also more gentle, puts less stress on the engine and read axle.
Also, its easier to get an auto gearbox to survive higher power.
I’ve just started making muscle cars motors, you can get the responsiveness into the 20’s-30’s you’ve just got to drop the fuel ratio into the high 12 low 13 range, jack compression and spark. 1970 was a prime year, just before the leaded gas started to be phased out and premium was still cheap. Don’t be afraid to put economy into the low to mid teens as those old iron beasts were not really known for efficiency. Here’s my thread with my first batch of Detroit iron, viewtopic.php?f=34&t=3621
edit also for a more responsive engine use triple double barrel carbs, for more power go double quad barrel.