Backspcing question for you big wheel guys

i have 17x8 cobra wheels for now that have 5.72 backspace. the fronts with that backspace would work fine for you but the backs need spacers to work with a stock dimension A-body rear end. mine's from a B-body so they fit perfect. I've seen other members cars here that are running 17-18 inch wheels with lesser backspace, maybe 4.5-4.75 that seem to have pretty good luck, I plan to go bigger but i won't spend a penny on new rims without making sure they will fit first so i'm going to blow a few bucks and buy a Percy's wheelrite tool for gettin it right.
I'm hoping to squeeze a 9.5 wheel and tire on the back without spring relo or tubs.