What do you mean by "frame differences"? I think the undercarriage is the same from 65 to 66. I think the engine bays were the same width and the K-frames interchange. Valiant and Dart did differ in wheelbase over the years, but probably doesn't affect the front rails. The main difference I understand for 1966 was the move of the wiper motor to the engine bay, which surely affects the firewall, but that varies even in the same year between AC and non-AC cars.
