69 B body 8 3/4 in a 68 A body with 15x7s?

Your going to need more back spacing and a wider rim to run that wide of tire. Yes people put 255 on 7" wide rims but you should have at least an 8" rim for it to fit properly. Good luck finding a 15" rim with the extra offset. Centerline Racing and I think Weld are the only ones who have some positive offset 15" rim. This is why I went 17" on rims, way more tire options (i.e. better than the way outdated BFG's or Copper Cobras in terms of tread pattern, compounds, and cost about the same) and better custom wheel options. Some folks run a 94-04 17" Mustang rim, there is a large post on that topic with all the info you need if you go that route.

Get the rear in there and measure. Buy a Percys Wheel Right if you want it perfect.