8 3/4" narrowing - Dimension Help

If you simply want to confirm what a stock A body 8.75's dimensions are, you need go no further than the sticky thread at the top of this very sub-forum.
An accurate 8 3/4" rear axle width list
If you stick with the factory 8.75 dimensions then Dr. Diff's axles will fit fine. Anything else and you're into custom axle territory.
And yes, factory A body 7.25s are a different width than A body 8.75s.
P.S.- If the 8.75 housing that you're starting with is from a B Body, chances are you only need to move the perches and use wheels with a bit more backspace. It's done quite frequently with perfectly good results.