Formula/LS1

I have a 2001 Formula and purchased M/T 275x60x15's and Weld Prostars 15x10 with a 7.5 backspace. I followed directions from a LS1 thread on grinding down the caliper to fit these specific rims. Well needless to say I grinded till the caliper is just about ready to break in half and the wheel still rubs on the inside. Does anyone have an idea what I'm doing wrong? I spent a lot of $$$ to turn back now. Thanks. Any recommendations would be greatly appreciated.