Exactly.
15x7's with a 4.25" backspace will be a better fit. The 15x8's with 4.5" of backspace will be closer to the fenders, just like your math suggests. The 8" rim is an inch wider, but only has an extra 1/4" of backspace. So you lose a 3/4" of fender clearance. And you can only run 225's anyway, so you don't need the 8" wide rim.
I might add you don't need drop spindles with 1.09" torsion bars either. You can lower that car as much as is practical with just those bars and a smaller lower bump stop, and you'll have better suspension geometry than with drop spindles.