9in rear drum rakes; do 7 1/4 and 8 3/4 interchange???

When you swap out the front 10in drums I will take them off of your hands! LOL

Guys, I don't know what was done, but I bought my 69 GTS well used and altered back in 1980. it has 9" brakes on the 8.75 rear. I have had it stored and am just now beginning to work on driving it. it has the 10" fronts which will go away for a 73-76 disc swap. I will need new rear axles for the 4.5 BBP and am in need of ideas and advice on brakes. to use something like Diplomat brakes from an 8.25 rear do I order the stock machined axles in 4.5 or do I need to specify a special axle length or bearing surface to fit the backing plate depth? thanks, paul