YET another will it fit..... steelies on a 68 Val

Offset is measured in mm's. So a +6 offset is a 6mm offset, or about a 1/4". But offset is always listed in mm while backspace is listed in inches.

Offset is useful because if you keep the offset the same, the centerline of the rim stays in the same place even if you change the width of the rim. Backspace on the other hand changes, a 15x7" rim with a 4.5" backspace has a +12 offset, where a 15x8" with a 4.5" backspace is a 0 offset, and the centerline of the rim is a 1/2" different between those two rims.

On that note, a 15x7" with 4" of backspace is a 0 offset rim again, so you'd move the wheels a 1/4" out toward the fender. And that would probably mean you wouldn't be able to run a 225 anymore up front.

If you want to run the same size wheel all the way around, you're pretty much stuck with a 225/60/15, and there's nothing to be gained really from running a 225 on a 15x8. The rim width range for a 225/60/15 is 6-8", and the measured rim is usually a 6.5", so even a 7" is wider than what they use to get the tire specs. And a 15x7" with a 4.25" backspace would be fine for that. If you could find a 15x7 with a 4.5" backspace that would be better, but that would probably have to be a custom rim. You might be able to run a 235/60/15 if you could get a 15x7 with a 4.5" backspace, but in the front you could run into issues at the lower corners of the wheel opening because of the height of that tire, especially on a Valiant. Some modifications at the lower corners might be needed.

And this is why 17's and 18's make so much sense. With a 17x8" and 5.25" of backspace most BBP A-bodies could run a 245/45/17 all the way around with pretty much no modifications at all. Cars with 8 3/4's and BBP axles probably wouldn't even need a spacer in the back, 8.25 and 7.25" cars would though. And a 245/45/17 is a pretty popular size, so, lots of tire options. That's probably exactly what I'll do with my '71 GT, if I can stick with the plan and NOT do a 1/2" spring offset on that one.