I have 245/60x15 on the back of my Valiant, they are 26.5" tall. I believe you can go up one size in your Dart, to a 255/60x15 with no issues. With the right rims and backspace, I think you can get one more size to 275/60x15.
Show us some pictures! I am in the hunt for a 69 Dart myself, just can't seem to find a good one.
