Well, the 68 I bought has those funny little drag wheels on the front and since they are pretty squirrely in the streets I thought to change mine out too. I found a site on the internet (unfortunately I didn't bookmark it and have forgotten what it was, but it involved Car Craft Magazine, I think) and they listed a bunch of car/wheel sizes from Weld Racing. Weld gives the fgf either a 14x8 or 15x8 as the correct wheel size with the 4.5 in b.s. for both front and back.
I'm sure others will give you more information that I as I have only been involved with this for a short time.
This is exactly the combo i wish to run on my 68 bird, 15"x8" wheels with a 4.5" backspacing. I need to know if they will fit on the front as well as the rear with a 12 bolt rear end, also, is their enough wheelwell clearance to run 275-60's in the rear and 235-60's up front?
Anywho, I'm running 17x7 with 3.875 backspacing and they are right on the money. (225-55ZR17). Maybe not be the look you desire, but thought the calculator might help you decide.
One way to go is to have a set of wheels and tires on your car that are of a known size and back spacing. You can measure around them and see how much room you think you have. Then you know how much to add and subtract for your car because cars are all different depending on the particular suspension and body ect.
I have run 15x8 Snowflakes on my 68 before. 235/60's on front, 275/60's on back. The front had slight rubbing on the bottom of the fender lip. The next smaller tire size would probably be fine.
