What's the axle center register size?

What's the size of the center register on large bolt pattern axles? I assume it's larger than the small bolt pattern. Do other axles have different register sizes? Let's say one gets a set of 11" backing plates off a 9-1/4" diff, would the drum be too big? I got a set cheap, but the register looked huge, so I thought I would try the backing plates with a set of '60s C-body drums. Maybe it was my imagination. I'm used to looking at the small bolt rears.

All RWD Mopar 5 x 4.5" axles/hubs use the same register size.