Biggest tire on a 70 Challenger R/T questions

My project 70 Challenger is already mini tubbed & the leafs moved "inside" of the frame rails. I have 15" x 12" widened rallye's with a way incorrect offset with a stock length B-Body axle under it now with 29.5 x 10.5W M/T E.T Drags under it. I am still debating to re-do the relocation differently (the more common way)& leave it a leaf car, or ladder bar, or just go all the way & four link it,but leave it small tire. Question is what is the largest wheel tire combo I can still fit under it with stock frame rails if I leave it leaf car with a narrowed Dana & Doctor Diff offset kit? I plan on running in the Heads Up 10.5 Outlaw & Drag Radial classes when finished.