What size 17 in rim will work on 68 Dart Converitble?

If you want a 245 on the front with 17x8's and a 73+ BBP disk brake set up you need a minimum of 5" of backspace. Minimum. That would put the outside of the tires even with the 275's on my car, and I don't have any extra space. With a 4.5" backspace the tires would be touching the fenders, no clearance at all. Most 17x8's can be run with up to about 5.6" of backspace. There are quite a few members running 17x8" bullit rims, those have a 5.72" backspace and usually require a 1/8" spacer to clear the suspension parts.

245/40/17's are really short too, that's only a 24.7" tall tire. 245/45/17 would be a much better fit at 25.7".