Watching the HorsePower TV episode where they're building a track car out of a '67 Dynacorn shell, and they have an interesting way to measure wheel fit. DVR it if you can, but a little summary:
1. Find out your wheel backspace.
2. Take your tire diameter and cut it in half, getting the radius.
3. Take a long piece of wire and make an "L" bend in it at the backspace point - say, 4.75"
4. Attach wire to the center of the wheel hub, (with the wheel off) at the radius distance from the "L" bend - tape should be fine.
5. Turn the wheel to check the interference of the "L" bracket and at various positions.
6. Do the same the opposite way for the fender lip clearance, but subtract the 4.75" backspace from the total width of the wheel. An 8" wheel is usually 9" wide, so 9-4.75=4.25".
I can take a pic from the Horsepower TV episode if you want . . .