7 1/4 drums fit on 8 3/4?

The backing plates will interchange from 7 1/4" axles to 8 3/4" axles, make sure the drum Dia. matches the backing plate dia. (IE: 10'' backing plate = 10" drum). also the bolt pattern is important, depending on year... all 8 3/4 A body axles had a 4" bolt circle and I believe a 10" drum.
The 7 1/4 " was made in many variations.
To answer your question correctly... It depends on many factors.
But in your case, it should all work out.