You have me wondering: you have a ’74 Duster with small bolt pattern wheels?
From the early ‘60’s, there were 5 different A-body differentials used:
’63-72 7¼” 53.20” flange to flange (backing plate to backing plate) (small bolt pattern)
’66-71 8¾” 52.23” flange to flange (small bolt pattern)
’72 8¾” 52.578” flange to flange (big bolt pattern?)
’73-76 7¼” 51.40” flange to flange (big bolt pattern)
’73-76 8¼” 51.40” flange to flange (big bolt pattern)
All A-body differentials are 43” perch center to perch center.
FMJ differentials are all 54.340” flange to flange. 44.46” perch center to perch center.
The perches will need to be cut off and replaced or re-welded on.
FMJ perches have a larger center hole – so if going into an A-body, then use perches with the smaller holes when re-welding them on (new ones can be bought fairly cheap).
Also note: the 7¼” shock plates will be different – so one will need to find either 8¼” or 8¾” shock plates, as well.
BudW