Drivetrain questions - 64 Dart

Ok, I’ve done some poking around online, in these forums, videos, etc. I do not understand how you decide what rear end to run, what gear ratios to choose for the rear end, the actual practical effect various ratios have on performance, how transmission ratios work, the benefit of overdrive/gear splitting.

Please help me understand. I’ve got a 64 Dart 270 with the 225 slant 6 and a 904 push button controlled transmission. Disc brakes are few and far between for the 7 1/4” rear end, but plentiful for an 8 3/4”. If I switched to an 8 3/4”, what kind of gear should I be using in it? I don’t race it, but I’d like to feel like there’s a little more power going to the ground than whatever is in it right now. I do a lot of city/interstate driving and my biggest complaint is that around 65-75mph I dont get any real power I don’t feel like, and it uses a lot of gas to go that fast. I daily the car, and take the interstate everyday.

Is this something where an overdrive gear is helpful or am I misunderstanding the use of overdrive?

Any help or tutorials is appreciated. Y’all have experience here, I don’t