What is tallest and widest Nitto or BFG drag radial that will fit under rear fenders of 93 fox body mustang? Planning on Center Line Auto drags 15x8.5 with 4-7/8 back space. Been told 275-50. Also told 295- 50 with 4-1/2 backspace will fit. The car is daily driven, lowered 1-1/2 inches. Planning on removing quad shocks and rolling lip if needed.  

275/50 would fit, no problem.  Since you're ready to remove the quads and roll the fender, the 295's would work with the backspacing you mentioned.