[AROWHED]

I would like to see those photos for sure.

To the best of my knowledge, and this applies to the rear end only for 65-66(and maybe up to 1970?) B-body Pontiac, I've come up with this....

To perfectly CENTER the wheel in the wheel opening between frame and quarter lip (which may require trimming with the wider rims anyway):

A 6" rim must have 4.375" (111mm) backspacing.
A 7" rim must have 4.875" (124mm) backspacing.

Obviously 6 and 7" rims need not be perfectly centered due to being so narrow already.
But centering is critical as we go wider....

A 8" rim must have 5.375" (136mm) backspacing.
This is exactly what I have right now on my 65GP, the rims modified by Stockton. Very tight fit with 285/70 tires...had to roll the quarter lip up. Don't run fender skirts anymore, too painful to deal with.

A 9" rim must have 5.875" (149mm) backspacing.
A 9.5" rim must have 6.125" (155mm) backspacing.
A 10" rim must have 6.375" (162mm) backspacing.

So while all these rims widths change, the thing they have in common (for perfect centering) is their "offset"....it always stays at 22mm, about .9". You can verify that on that tire sizer site... http://www.rimsntires.com/specs.jsp

I see on Ebay Motors that for at least the MB 17 x 9.5" wheels, they tend to have a 6mm offset, that is just not enough..with the factory drum brake 65.50" wide rearend, it's won't allow your wheel\tires to be centered between frame and quarter lip...pushing them over half an inch closer to the quarter lip, and away from the frame.