Tommy, man its been a LONG time since I had to even think of what the BS is on my 15X4 bogarts, and unfortunately they are on the car right now (they've been on there since I blew it up). Out back you will want something around a 6.5-6.75" BS. My 17X10.5" wheels have a 6.75" BS and they sit fluch with the fender, you may want to keep the BS similar to mine (maybe a hair less) and move the wheel in just a hair out by the fender so you don't have the tires hitting the fender when the car squats. I think something between 6.5" and 6.75" would work our great.