Right, and a 15x7 with a 4.25" backspace has the same centerline as a 15x8 with a 4.75" backspace. But he has BBP axles so add 5/16" for the increase in track width. And he has a 3/4" offset so add another 1/2" (yes, I know I added a 1/2" but even the 3/4" offset only offsets the front hanger a 1/2") . Now the backspace is at 5-9/16".
I run my duster with a 1/2" offset and about 5/16" of clearance to the spring, it just barely touched the inner wheel well when I was cross-axling speed bumps before I added the rear sway bar. Inner tub is the same on both of those cars. He should be able to manage 5 5/8" with the measurement he has. I wouldn't run the spring clearance that tight on a 275/60/15, my 295/35/18's have like 2.5" less sidewall to flex.
