Rear number 1
You can look for an 8.75, or 8&3/4 but you will spend a lot of coin. I didnt have one, and at the time i was collecting parts to build one, i didnt know about the alternatives. When the smoke cleared between swapping and trading i am into an 8.75 with new BBP axles rebuilt 10x2.5" finned drum brakes, new 3.73 gears, new bearings, new locker center section, new weld on axle perches for $1,200 and today thats actually a deal. If you can find one thats for an A body complete drum to drum for $500-600 needing rebuilt, thats a pretty good price these days. Bring a tape measure, and know where to measure and know your dimensions so somebody doesnt tell you its an A body, but it ends up being a B or C body.
A body 8.75 rears are also small bolt pattern, so you will need to either redrill the axles, find C body axles that arent necked down and have em cut and resplined, or buy dr diff axles for A body, new in BBP, with new bearings for $300. Your rebuild BBP brakes on your 7.25 will however move right over to an 8.75. I know i used mine off the 7.25 off the 74 dart parts car i had, and installed em on my 8.75. Any way you slice it, you will be out some money on one of these. The plus side is they are very strong like a 9 inch ford is.
Rear number 2
8.25 or 8&1/4 . This is the one i shoulda looked for several years ago. Already BBP its mopar to the core and in an A body, its the correct width and a direct bolt in. Your 7.25 BBP rear brakes will swap to this axle too. Typically $200-$300 for one complete drum to drum is normal. You can get gears and a sure grip out of jeep grand cherokees at your pick n pull yards. Should be a tag on the rear end of those vehicles attached to the diff cover with gear ratio, and if its a sure grip or not. Add in used gears, a suregrip, and somebody to install and set it up you might be out the door $600
Rear number 3
My new fav. Ford explorer sport trac (the pickup truck explorer) 8.8 theres a few issues with these but the plusses outweigh the minuses. First the plusses. Its newer, the 8.8 has a bunch of gear ratios available for it. Its extremely durable. They are all over the junkyards at this point, they have the same BBP as a mopar, bonus it has disc brakes on it for that sexy 4 wheel disc brake setup everybody wants, it comes with 3.73 gears , it comes with a locking diff, and they can be had for about $200.
The minuses are you need a bastard u joint to attach the chrysler driveshaft to the ford rear end yoke (minor annoyance) pumpkin isnt centered longer on drivers side, and wider overall to A body rear by 3" . The fix is to buy a second passenger side axle and cut and shorten the drivers side axle tube 3" and jig weld the tube back together making sure it stays square, and use extra passenger side axle on the drivers side. This centers the pumpkin, and makes it the correct width for a A body. Axle tube diameter is 3.50" this requires you to use explorer u bolts, and either slot your shock mount plates, or buy new ones from speedway motors or other stockcar parts places. You will also have to cut off the explorer spring perches, grind the tube smooth, and use new spring perches for 3.50" diameter axle tube about $25 for the pair. These need to be located for A body width, center, and pinion angle before welding in. The reason this is my new fav even with the fabrication work needed to install it is this. If you do most of the work yourself, and get a sport trak axle its got all the goodies and disc brakes to boot, you can buy and install it for about $300-$400
