7 1/4 rear end axles

Can you help me please with a question.
I have a 65 Dart with 7 1/4 rear end and small drum brakes and small bolt pattern lugs. The front is converted to disk brakes and large bolt pattern.
The car came with a spare rear end (also 7 1/4) with larger brakes and LBP. Don't know the year but the center section seems shot (water got into it). The axles are good.
Can I change only the axle shafts along with the drum back plates and the pressure plates (what keeps the axle bolted to the housing). It seems these pressure plates on the old axle have 4 bolts, and the new one has 5 bolts.
I am thinking on putting new bearings on the new axles and at the same time change the pressure plates - use the 4 bolts plates from the existing axle to fit the old housing.
Is that possible, is there a difference in length, any other obstacles?

Thank you,