In my opinion the 1st generation is 1972-1993. Forget Ram and Cummins. The frames were all very close, wheelbases were the same, cabs, boxes and sheet metal could be swapped easily with minor modifications. Bumpers bolted on and plow frames could be used throughout the 21 years. That's my take on it.
While I agree with 72-93 being all " the same" generation
But I don't agree with that being considered "1st" generation, as that is to say that Dodge didn't build "any" trucks prior to that????? BULL$#1T. All the more reason I wish I could afford to go find me a solid old "Dodge Brothers " version on which to restore. So I could then show everyone what "1st gen" Dodge trucks really were. Or at least find me a a 64-71.