narrowing rear question

I have an 8-3/4 for my 68 cuda that was originally from an a body and is still the full width.
it is at the guys shop who builds rears now.i would like him to finish building the rear before I finish the car and inboard the springs.
do you think I could just have him narrow it 3 in per side before I actually move the springs in or will I actually have to relocate the springs and take a measurement.
its going to be a while before I get to move them.
The 68 dart I had I moved the springs in & took 3 inches off each side plus mini tubbed it. All depends on what size wheel & tire you want run