I just did this on my 65 dart. I pull the tire and set the car on a jack stand under the axle leaving it at ride height. Get a yard stick and tape it flat to the disc or drum run it around in a half arch and find your maximum height. Then use cardboard over the end of the yard stick to find your maximum width. This will find the fender lip and or curve of the fender ( as in my case the fender curved inward minimizing my width) and be sure to clear the springs and spring clamps. Tape all this together and pull it out. Measure it for your max tire height and width and off set for your rim. Mine came out to be a 215 55 17 tire on the rims I chose
