Trying to figure out the wheel size I need to order.

if you cut it down know that real A-body axles wheel mounting flange to bearing face is different than on a B-E body axle. I believe that affects the backing plate and drums depending on what breaks you go with.

If you don't have too much into it you might want to look for a real A-body rear housing and axles. I have been told that 7 1/4 - 10 x 1 3/4 brake drums and backing plates swap on to a real A-body 8 3/4 housing with real A-body axles.