235/60/15 will fit perfectly on a 15 x 7 rim w/4.25" b/s.
245/60/15 might fit w/the correct b/s and rollin the wheel lip.
A 255/60/15 will not fit unless you install the offset kit or relocate the springs.
BTW, the 70 and up Darts wheel opening is different from the 67-69. A 255 tire will fit the 70 and up Darts, not a 67-69.
