That's pretty impressive! On my former 1970 Australian VG Valiant hardtop (basically a 67-69 Dart and exactly same sheet metal and wheel wells as any 67 up dart or scamp) the best I could do was 245/60 x 15s (BFG TA's) with the rims sitting in close to the springs and they still rubbed the wheel lip a little. I would have said 235/60 x 15s would be the largest to fit on these cars without interference.
I would however say that there is no way 255/60s would fit on the front, with rubbing on the wheel lip a real problem especially when turning at slow speeds and negotiating any little bump in the road .... even 225/60 x 15s would be pushing it in my experience. 215/60s or smaller for mine.
