Swapping rear end question

The 8 1/4 should have a tag bolted to the rear end with the gear ratio, assuming it hasn't been removed in the past 43 years. My '73 Dart's 8 1/4 had 3.21 gears with no sure grip.

It should be a fairly straight forward swap. The only big thing I can think of is the driveshaft. If you get the driveshaft from the '73 Duster, you may or may not have to change the driveshaft yoke and u joint. The 4 speed and 727 use a large yoke, and the 3 speed and 904 use a small yoke.