wanted 8 3/4 rear end

Why not use the 66 Charger 8.75?

It's 1 3/16" wider on each side than the A body rear. Grab wheels with a bit more backspace. Plus it will be big bolt pattern. Move the perches in 1/2" on each side or spread the springs slightly.

Unless you are using small bolt wheels, I'd suggest to use what you have.