I have various wheels I'm considering for the front of my 67 Barracuda, and I'm trying to figure what tire size will fit for each rim. SBP with KH brakes. 14x6 Cragars, 4.125 backspace, currently with 215/70/R14. Crusty, rusty. 14x6 Cragars, a smaller backspace, 3.75 or 4, currently with 205/70/R14. Like new, although I'm not a fan of the backspace. 14x7 chrome, not sure of backspace, bare rims. Rusted centers, but may paint black with dog dish. I also have a set of sorry Keystones I'd like to use, but I'm not sure if they are safe. How do you calculate the best size that will fit? Also, I have one 5.5 wheel for a spare, what's the smallest tire you can mount on a 14x5.5 wheel?