with a 275/60-15 on an 8" rim, you ideally want 4" of backspacing.
If you are running welds, 3.5" will require rolling or cutting the fender lip and will limit lowering the car.
4.5" will rub on the frame. You can can run a small spacer or notch the frame to the seam and take a BFH to the wheelwell.
My fathers car had 4.5 with the small notch BFH. This picture has the quick time pros which are a little wider than the 275/60, but pretty close.
Here are some links to shots of it with a 275 radial
http://bkdigim.exposuremanager.com/p/msmt1018/esp1018-074_40_1
http://bkdigim.exposuremanager.com/p/msmt1018/esp1018-074_40_1
http://bkdigim.exposuremanager.com/p/mst920/esp920-132_38_2
http://bkdigim.exposuremanager.com/p/msmt906/esp906-085_36_2
http://bkdigim.exposuremanager.com/p/msmt906/esp906-113_36_2
http://bkdigim.exposuremanager.com/p/msmt906/esp906-128_36_2