15x8 Backspacing for Fabtec lift.

Could anyone please tell me what kind of back spacing I would need to run on 15x8 on a 6" Fabtech lift, I also have a 2" body lift. I have 15x10 right now simulated beadlocks black rims with the "D" type holes, but I had to put on a spacer in order to clear the calipers which I dont want to do. I want to run 15x8 for tha taller skinnier look, also a lighter rim these are for a 35x12.50 tire. I would like to buy black rims again but the ones with the larger holes all around, 4 wheel parts has them (series 97) but no one seems to know what would actually fit. The manual on the fabtech lift says to run 3 3/4 backspacing on a 15x8 with a 35" tire, but I heard of members before having to run a spacer with this backspacing.
Any help would be greatly appreciated.
Thanks.....