I know this is a little late and also a different year, but I kept the stock opening on my 69 and use 32 x 14 Goodyears on 15 x 14 rims w/ 4.5 offset and all I do is let the air out if they need to come off.
I'm in the process of installing Harwood stretched wheel well openings on my 67 Camaro. It has 31x16.5-15 ET Streets on it. The old quarters were stock with the front and rear of the openings stretched out in a sort of hillbilly fashion. It didn't look bad from a few feet, but up close they didn't look that good. So now I'm installing these, and think I might step to a 33x18.5-15 ET Street now, the car needs some gear taken away anyways (has 4.88s and runs out around 1000ft on spray) and I think 33s will fill the wheel well a little better.