Parts Swap Between 69 and 75 Dart

if it has an 8 1/4 (8.25) you might use that. Nothing wrong with 8.25, they are tougher than most give them credit for and gears are cheap for them and easy to find because they used the Chrysler 8.25 in a lot of jeeps and it will be the big bolt pattern to match the disk brakes.

If it is a 7 1/4 keep looking for something else. IMO

those are your only 2 options if it is original