I have changed tire sizes a few times on my 70 Challenger. Up front, I started with 14"/225's, then 15" 235's, and now 15" 245's. I upgraded to 15"s to be able to run the Dr's 11.75" discs up front. Besides the tire size, you can gain some clearance with the torsion bar setting too.
The rears I started with 14" 245's, then 15x7" 255's, and now 15x8" 275's with no clearance issues with the stock rear setup.
What brand headers are you running?