It obviously can be done as many have gone down this road; that said it isn't a super easy and definitely not a cheap/quick effort. For me personally, I don't think I'd do it; if you just have to have a 70/71 then go out and find one (they are still out there), however I am a big supporter of "your car, do what you want". While the 72-74 Challengers are not as sexy as the 70/71, I do like them for what they are. I had a 74 with a 440 6 pack (obviously not original) which I loved. I would be proud to have one again, actually I looked for a 72-74 4 spd car for quite awhile but never landed one.
Again, your car, your money, do as you wish. But, I would say there is no shame in owning 72-74 Challenger.