OK, I get some different numbers (not trying to be argumentative). The original poster did not state what size 15" tires they planned to run on the 8" wide wheels. However, I'm assuming it would be "around" the 225-60-15" (roughly 26" diameter) I have mounted on 8" wide wheels - and thus will use then for reference. I briefly ran these wheels/tires on my Pinto and they are visible in my side image.
As I stated above the rear springs are not parallel. The spacing is wider at the rear, therefore bringing the spring closer to the tire - the point of minimal distance. With that in consideration I measured roughly 13" back (half the tire diameter) from the axle center line and got the brake drum face a hair under 6" off the spring at the tightest point. I also got a 5-1/4" backspacing on the wheels I am using for reference. When I installed the wheel I get right about 1/4" of clearance between the tire and the spring as there is an additional 1/2" tire bulge beyond the wheel edge. While I would call that minimal I don't recall rubbing and the tire shows no indication of having done so. As I said, I ran them briefly, a long time ago. Also my rear is a Mustang II, 8". I'm assuming all the Pinto/Mustang II rears are the same width. And, again this is rear leaf spring clearance issues. I don't recall any up front issues but it was some time ago. The tires for sure were close to the wheel opening and might have rubbed. Then again they were 225-60-15".
So, at least from the measurements I'm getting I see the following:
1. +30mm (positive offset) moves the rim 1-1/4" closer to the spring off the 4" centerline of a 8" wheel.
2. That would create the same backspacing of 5-1/4" like the wheel I am using for reference.
3. I have successfully had a 5-1/4" backspacing on a 15" X 8" wheel and 225-60-15" tires. I'd call it close, but I have done it.
It seems at least feasible that the +30mm 15" X 8" wheel will work. Jays79 YOU need to check (and double check) this for yourself. You also need to determine if roughly a 1/4" clearance at the spring is sufficient. Even if my numbers are correct it will be very close. BTW, there wasn't much tire clearance with the inner wheel well either. And, as the car tilts .. . You can use $pacers, but there is the cost and probably anything over 1/8" will require longer $tuds too.
Little things can add up. I reference 30mm as 1-1/4" but in reality it is a bit smaller at 1.18, not 1.25. That is favorable. Also if you run shorter tires, say a 205-50-15" the tire bulge will be closer to the axle centerline and have a slightly greater gap between the tire and the spring which is also favorable.
Anyway, I don't want to mislead here. So, if my evidence is in error please state so - and why.