[CMcAllister]

Typically, one sources the wheels to be used with the desired BS that will be used, throws a set of the largest (or desired size) tires that will be used on them. With no rear in the car, position them under the car in the desired location, measure wheel flange to wheel flange and build the rear to that dimension taking into consideration the brake kit being used, pinion offset, etc. Build the rear to wheels - not the other way around. Typically.

Enough of those cars out there, the dimensions and specs should be standardized with little need to figure it all out yourself.