67velle, I have 285's sitting in my rear wheel house right now, and it looks like they will fit with about .375"-.500" clearance on the back side of the tire and just enough clearance for the outside of the tire to clear the outside fender well lip.
I went with an 18x10 (and 5.5 BS) and a 305/45/18 tire in the rear. I had to roll the wheel well lips and message the inner wheel wells. No rubbing on either side now but it took some trial and error work.
I'm sure most 68-72 GM A bodies are close but I know even the 70 - 72 Olds 442s and Cutlass' are slightly different. Better to measure and then measure again...but I think "usually" a 295 in the rear and a 245 in the front (using 18x10 and 18x8 respecitively) is about as big as you can go without any mods. Going w/a 285/235 setup (or smaller) will just be that much safer.
Not sure if Pre 68 require less tire/rim or different BS but I did tons of research on the 68-72s before I got mine.
ZZ3 I don't know if this will help you but on my GTO I will be running 18x9 and 18x10 BOZE MESH rims, the fronts will be a 5.25 back spacing with a 275/35/18 they sent me a test wheel and I installed a tire and it fit great with no rubbing anywhere. As for the rear I am running a 295/35/18 with a 5.75 back spacing.
Brian Hobaugh SCCA National Tour June 2014
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