Need help for tire size

Egad! This is a tough one.

I haven't tried 18s on either of my A-bodies. However, I feel like I've tested the limit of what will fit under an unmodified fender. The two dimensions that make the determination is the rolling diameter and the width of the wheel tire package.

Noting what others have tried and my own tests, here's what I've seen. Front 235/60-15, Rear 255/60-15.

Here's how the critical dimensions compare. (tire, diameter, width)
235/60-15, 26.1", 9.5"
255/55-18, 28.9", 10.1"
255/60-15, 29.1", 10.2"

IMO the 18s will work in the rear. The front fenders will need to be rolled under for clearance. The 235s with a 7" wide, 4½" backspace are a snug fit under the front fenders. If you have a bud that'll trade you a pair of 255/55s for maybe a pair of 225s for the front, that may be a plan.

FWIW, these tire dimensions are available from the manufacturers' web site.