I've read most of the threads on wheels and backspace. I've verified that an 8" width wheel will require a 5" backspace. I tried an 8" Crager SS wheel with a 4 1/4" bacskpace and P275/60 x 15 tire and it would not clear the outside of the wheel opening with upward axle travel. A 5" backspace wheel likely would clear. A P225/70 x 15 on a Crager 7" wide wheel with 4 1/2" backspace easily clears. The problem is the only 8" width wheel I can find with 5" backspace is the factory type Rally I wheel from Wheel Vintuques. Not a bad choice but I would really like a more aggressive period wheel such as the Crager or American Torque. However on their websites I can not find an 8" width wheel with 5" backspace. Anyone know of a wheel source with 5" backspace?

Try a different aspect ratio tire, the /70s will not work on my '67 but the /60s fit.
I have a set of Redlines with a 70 aspect on Rallye II rims that rubbed in the front and rear, I had to take them off and go with the /60s T/A radials.

Have owned a few '67 GTO's over the years that had the bottom of the rear wheel opening moldings trimmed nearly an 1" so 29" slicks and big sticky MT street tires could fit in the rear wheelhouses.

If there is a certain style of wheel that one is totally stuck on using, can also examine the spacing mounted tire/wheel combo in the wheel house, and then if it solves the problem, consider going to a 1/2" narrower on each side factory rearend, like the '65-67 Chevelle 12 bolt, or in a rearend build, having the housing narrowed slightly in a jig. In the 90's, built up several '65-67 12 bolt's specifically for '66 and 67 GTO's. Over the last 10 years, have narrowed a few 8.5 A body housings and installed A10 housing ends, so the customer could have the correct width and the big plus of the largest tapered axle bearing/thickest axle.