will B or C body 8-3/4 fit under a 73 duster

65-70 covers 3 different widths for B body axles, and they vary by a decent amount.

I use a 68-70 B body 8 3/4 in my '74. How easy it is depends on the rims and tires you're running. If you're running stock wheels it's fine, you can run up to a 225/60/15 on 15x7's with 4.25" of backspace. If you're going to run 17's or 18's out back it's even better than running an A-body 8 3/4, most of the off the shelf offsets will work just fine. The only real issue that comes up is running 15's with a really wide tire. Once you go back 15x8 it gets hard to find enough backspacing with off the shelf rims to deal with the wider rear axle, the '65-'67 B body rear would be a better choice if you want a meaty set of tires on 15" rims.

C-body depends on the year, a 65-69 C-body 8 3/4 would probably be possible but it's not a good idea. You'd need a lot of backspacing. The later C-body 8 3/4's would be a bad idea unless you're planning on having them shortened.

Here are the dimensions of all the various mopar 8 3/4's. All of them will require the perches to be moved.

An accurate 8 3/4" rear axle width list