Brian, first, the drums are usually only 1/8" thick at the flange, so you need to dial back your calculation a little.

It's not uncommon to put a maverick rear under these which measure 56" mounting flange to mounting flange (with drums on). So looks like you're in good shape. 4 3/4 bolt circle is "standard" Chevrolet pattern, so as long as the drum doesn't do anything funky shape wise then lots of options out there. If your measurements are accurate, you should be able to use a wheel with up to 1" positive offset, but it's always a good idea to do a mock up to verify.