A question I should of asked before

Yes A Body 8 3/4 axles will fit inside the narrowed B Body 8 3/4 housing as long as the narrowed housing is narrowed to spec width wise.

Like those 8 3/4s, Beef, Pumkin comes out the front.

Might look onto if you are going to use the A Body Drums and Brakes with your A Body axles, this will work and ends up small bolt pattern.

Or the B Body Drums and Brakes and going to the 4 1/2" Bolt Pattern axles. This you will want to look into for fitment. I am sure others on here can give you guidance on this. Dr Diff too . . .