cut out fender flares

Backspacing How much do I need in my tires to be able to fit some 33×12.50×15 mud tires on my Jeep Cherokee 91?
It is one 4DR, holds a 4 "lift and I have the cut-out Bush wacker fender flares. For the population now has 15" tires. I am looking to get some 15×8 wheels.
Stock wheels will work well. backspacing on a 15×8 standard will also work.
K10 Great Project Suburban 86