help with wheel ofset

I currently have those same 15x7 ultra rims on my 69' barracuda (rear) with 255-60-15's. BFG specs that tire at 27.1" x 10" section on 15x7 rim. This combination works fine with about 1" clearance all around on stock setup on my barracuda. The tires are a bit baggy on the sides and even though i have tried different tire pressures, the tires still wear a little too much in the middle for my liking due to that being the max size possible on that rim.

In order to improve this I actually just picked up a new set of 15x8 of the same rims. 255-60-15 on 15x8 should actually increase section width to approx 10.4" as well as be a better fit for those tires. Id like to go 275-60-15 however its very doubtful it will fit due to the 15x8 having same backspacing as the 15x7 version of those wheels... anyhow im going to the tire shop today to check out new tires and will post with what I find out.