20x12 wheels

[540z28]

Working on a 69 Camaro with my son. It has been mini tubbed and will be running Ridetech rear suspension. Not planning to do any heavy track use, mostly a cruiser. I have a set of Billet Spec 20x12s that are left over from another project. I am hoping to use them. We used a set of 335/25 20s off of a ZR1 to check fitment. Sure was tight. about a 1/2 " or so inside and outside. I do have room to cut and roll the fender lip about 1/2" to gain some clearance.
However, was looking at options to still utilize the wheels. Looking at 325/30s and see they could go on a 12" wheel. They might gain me 1/2 " extra clearance. 315/35s gets me even more side clearance and could go on a 12" but the diameter really jumps like 2 inches over the 335/25s that are 26.6 in diameter was using as a reference.
So my questions are anyone has experience with 315/35r20s on a 69 and does a 325 gain 1/2" more room than a 325?.

Thanks.

[Forgeline Wheels]

540Z28, I would stay away from the 315 tire. The sidewalls are going to stretch too much for the musclecar look that I am sure you want. I would recommend that you look at the specs on each brand of tire that is available in the size that you decide on. Not all tires are created equal. Some tires run wide and some run narrow. Look at the section width as well as the approved wheel size(s). A 12 inch wide wheel is actually 13 inches total width. Wheels are measured at the bead where the tire seats and not from outside edge to edge. With that being said, look for a tire that has close to a 13 inch SW. This will keep the tire square with the wheel and offer the best look and the most clearance.

On a side note, 1/2" clearance on both sides is very common and should not be a problem. especially since you are using low profile tires. The sidewalls to not flex much.