Max tire size - 68 dart

There are tons of threads on max tire size for various different A-bodies and rim diameters. Make sure you look at threads regarding Dart's, because Dusters, Demon's, Dart Sport's and Barracuda's can all fit more tire out back than a Dart can.

As for tires on 15" rims on a '68 Dart- Yes, 225/60/15 will fit up front, you'll want a 15x7 with at least 4.25" of backspace to do it. In the back, with the stock spring locations, you'll probably want to keep it to a 245 at the widest. Some have managed to fit larger, but it depends on the car and how much cutting and rolling you're willing to do. As far as backspace goes, it will depend on which axle you have and what bolt pattern you're running. A-body 8 3/4's are not the same width as the 7.25" and 8.25 rear axles. And, going to big bolt pattern axles on the 8 3/4 changes the width.