all models of muscle cars start to decline in value after 71. That is is when the gov't polution controls started to kick in and the engines made less HP and the cars performance went down.
70-71 Cudas worth slightly more than equivalent Challengers, just because of lower production numbers and more recent publicity. I think the 72-74 Cudas and challengers are all worth about the same with the base engines and equivalent condition. Maybe 74 a little less valuable.