They won't swap as far as I know, the 7 1/4 is the odd man out.. The easiest thing to remember is that any big bolt pattern axle brakes can bolt to any other big bolt pattern axle. And keep everything as a system, don't try to mix and match drums and backing plates etc.
Try and ID the year/body style your 8.75 is from. Then get backing plates.
i.e.- if your rear is from a '69 Ply. you can get backing plates from a '69 full size Dodge that could be larger
which would allow larger brakes on your project.
well heres another problem.. My wheels just fit. I have a bout 1/4" of clearance on both sides. I know that from flange to flange my 7 1/4 is the same as an a body 8 3/4. If i use big bolt patter breaks, will it push my whees out?
The brakes won't give you the bolt pattern. Measure the distance from the axle flange to the housing flange on both rear ends. If they are the same then Moser can make you big bolt axles with a-body offset.