valiant/duster

Which rear end are you talking about?
The 7 1/4'' is virtually the same if small bolt pattern, the brakes may have some differences.
There are some mid '70s ('73 to '76) 7 1/4'' rear ends that were big bolt pattern and the brakes were different as well as the backing plates. (10'')
The 8 1/4'' rear ends were all 10'' brakes and big bolt pattern, and they were used from '73 to '76.
The emergency brake cables were also different on the newer cars from '73 to '76 as well compared to the '72 and older ones.
The cars with 10'' brakes are slightly wider than the 9'' brake ones mainly because of the brakes are wider and they required different backing plates and drums. Not really a substantial difference though.
The spring perch width is the same on both rear ends.
The drive shaft is shorter on the 8 1/4'' rear end cars.