[AROWHED]

I know this is a late reply, but I just recently registered!
I have a 65 GP, very similar to your 66 in this respect. Just shipped two 15 x 7 Rally II to Stockton Wheel to have them widened to 15 x 8, with 5.500" custom backspacing. These wheels will then be fitted with P285/70R15, which will have a section width just a hair over 11", and a diameter of 30". The car currently has 265/70R15, but I want a wider look than that.

Minor trimming on the inside of the fender skirts, rolling the wheel opening lip, and removal of the factory fender skirt latch and striker plate is mandatory. I now use spring-loaded 1/4" steel pins to hold my skirts in place thru the wheel opening lip, they are out of sight, of course, and a little difficult to reach. Also, lower shock mount must be relocated.. I drilled a new 1/2" hole for the shock stud that is about 1" futher inside on the lower control arm bracket than the original...it does cause the lower shock mount to not point straight backwards anymore, but it looks fine.
Also, as was already mentioned, it is difficult to imossible to fit a wide tire between the brake drum and wheel opening lip when installing \removing tire...this can be overcome by unbolting the lower shock mount to allow further axle drop.

5.500" backspacing combined with an OVERALL rim width of 9.00" (a 15 x 8 rim is 9" wide overall) plus one inch of tire sidewall bulge past the rim equals 11.00"..and the above mods should leave 12.250" between frame rail and wheel opening. There will be only .625" clearance between tire sidewall to frame, and tire sidewall to wheel opening. Poly busings in the rearend control arms will then help minimize tire rubbing. I beleive this is as max tire as you can get before major surgery on the frame is necessary. Will try to update this topic with pics once I get my rims back from Stockton.