you can get allot more tire & rim in a 68 B-Body, there are a ton of tire size threads here & in the racing forum too... I currently have Polished Halibrand Slots both front & rear on my 68 RR, 15x7's w/245/60/15 front, 15x8's w/295/50/15 rears w/BFG Radial TA's, no fit or rubbing issues what so ever, you can go much bigger even, the 68-69-70's have huge wheel wells, ride height & proper back spacing will dictate allot of what you can fit, the wheel well gets thinner/narrower as it gets toward the top... It ultimately depends on what look your after or how big or stock appearing your going for... there is a very good Percy Tire Fitment Tool sold by Summit Racing &/or Jegs for like $70, you can use it for many different size bolt patterns, rim sizes, back spacing, diameter, width &/or tire diameter widths/sizes, to get an absolutely perfect fit, with out guessing... IMHFO well worth the small investment, especially to get the proper looking & perfect fitting rim & tire combo, that will fill the wheel wells & look the best...