Drivetrain questions - 64 Dart

Basics= if you have a "choice" of rear ends, your concern is based on the strength needed to live under the power of your engine. More power/racing use= bigger rear end because it will handle the increased power demands. Your car never had a high performance engine available, so it was equipped with a smaller rear end and smaller brakes which was adequate for the intended market (little old ladies). These cars are now 50yrs old and people are looking to get more performance from these cars (God love them), so now you will see discussions about upgrades- brakes, engines, rear ends/ratio's. You may not "need" any of these things. If your car is not adequate for your needs, if you want to change things, then the folks here can educate and help you do that because they have done it.