how big a tire can I fit

245/60/15’s and 245/60/14’s aren’t supposed to be mounted an a rim narrower than 7”. Not sure if you’ve got 14’s or 15’s, but either way a 5.5” wide rim is too narrow for a 245.

With the correct backspacing most 71 Darts will be able to fit a 245 with the stock spring locations. But you have to measure your car, because the body tolerances have enough variance that it’s not a guaranteed slam dunk.