That's not entirely accurate. The 60 determines the height of the sidewall as a percentage of the width of the tire. The sidewall is going to be 60% of the tire width.Sorry but the height is determined by the 60 series the width is you 275. I went through this on my 68. I'm running 17's in front and 18's at the rear. My 68 had 14" with F70x14s from factory.
Just a comment. On my drum brake equipped 67 the distance from the drum brake mounting surface to the tie rod is 4.8"The 15x8 up front would be the most critical, you would want 4.75" backspacing to center the tread in the space you have.
I looked on Jegg's page for the wheels you mentioned but I do not see them.. Are they GM type rally wheels?every one may have a different experience but this is mine. Factory Rallys have 4" back spacing. I have 15x8 Corvette Rallys on my 67. 21560r/15 bfg on the front. they do rub when you go through or over a bump. I put some Koni's and Hotchkis springs on and they seldom rub. But the 215's will look silly on the back. I have 255/60's on the back. trimed about 1/8" of the wheel well and they fit pretty good.
Phase two: i am putting disc brakes on the front which set the wheels out another 7/16" or so which is now too far for factory rally's. I ordered 5" backspacing 8" wide rallys by Wheel Vintiques from Jegs for $82.00 each free shipping. So far they clear the a-arm and tie rod. Ordering 2 more for the back. Lots of clearance and it looks good. Summit has them for $78.00. For the price you can't clean up and paint old ones.
Thanks Garth,Factory wheels will have a stamped code by the valve stem hole, however I'm not aware of a factory 15x8 with 4.25" bs. You can use them, depending the tire size and suspension set-up.