Andrew,
You can fit a rim with 5.5" BS on the rear but do not want to go past 4.5 on the front
Ideally IMO the perfect rear wheel combination is a 15x8 with a 5" BS and a 275/60/15 tire. Wheel Vintiques sells 15x8 raqlleys with 5" BS. I have 2 of these and used them to run BFG Drag Radials 5 years ago. They fit perfect with a 275/60/15.
Right now I run 15x10 with 5.5 BS to fit as big a tire I can under there. I did have to trim the lip and the chrome moulding to fit the 13.5" section width in there.
a 225/70/15 on the front works great with a 15x7 ralley with 4 1/4 to 4 1/2 BS
If you have anymore specific questions please don't hesitate. I have used just about every combination of rims and tires on my Montes.
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'71 Monte - NA, 10.78 @ 125mph, 3.42 gears, pump gas driver.