The backing plates are different between the two. The 7 1/4 axle has 4 bolts for the shield while the 8 3/4 has 5 bolts. I'm not sure about the brake shoes. The wheel cylinders are different too.
74, I am not sure about but 72 back will work with no problems.
I have 10" they work fine. Just drill the missing hole in the backing plate. I wouldn't use the 9" personally. My wheel cylinders are the same on my 10" 7 1/4 and my 10" 8 3/4.