swapping out ring and pinion 7 1/4

'70-'71 FPC says the 7-1/4" differential case is 2404 193 through "approx Aug '71" (then 3507 166), and the differential side gears are 2404 194 right on through.

'72 FPC says the 7-1/4" differential case is 3507 166.

'73 FPC says the 7-1/4" differential case is 3621 981 (side gears still 2404 194)

Given Chrysler's tendency to assign new part numbers to fit-and-functionally identical parts, this doesn't definitely answer the question; maybe give Dr. Diff or Randy's Ring & Pinion a call and ask.