Taking a poll about which drivetrain I should use

I agree w/ option 4. This assumes the 74 Dart is being parted. If so, save the front suspension and disk brakes and the rear-end from the Dart for a later upgrade. Keep the 70 original since it adds much more to the value than you could get from selling it, assuming you have room to store it.

If you are also installing the rear-end from the 74 now, I assume you know the wheels have a different bolt pattern, so you need 2 spare tires. In that case, better to bite the whole enchilada and install the 74 front suspension now too.