ok, you have an 18,000 lb car and you want quicker acceleration and still want to be able to cruise for days at 75 mph.
the best way to do this is to run an engine with bigger displacement . . the bigger the displacement, the faster it will accelerate.
you will accelerate much more quickly if you change your rear gear ratio to one that is numerically higher, however, that will increase your engine rpm at freeway speed.
if your gears are currently around 2.80, i would go to at least 3.00 for your app no matter what engine size you use . . 3.23 is the numerically highest i would use.
going up 1 gear size with your current engine will be slightly noticeable . . going up by 2 sizes will be very noticeable but still wont be a drastic change in acceleration feel.
going up 3 or more sizes will be a drastic change in acceleration feel.
heres a very cool an rpm vs mph calculator you can play with . . enter 1.0 for trans ratio and 2.80, 3.00, and 3.23 for gear ratio and it will compare all 3 at the same time.
RPM/MPH From Gear Ratio Calculator by Wallace Racing
you absolutely must get rid of those horrible exhaust manifolds if you want to increase power . . you can buy shorty headers or old cast iron factory 427 headers from a 64 top oiler etc if you can find therm . . you may also be able to use 428 cj cast iron headers but the repos are $1,000.00.