The thing is, mopar people(& pontiac people) seem to have the highest concentration of this insane purism. I mean i get it if its some rare package, but most of use a putting together cars that originally came with slants or 318s.
I think the problem is you're being a little too rational here... We're talking about classic muscle cars here not modern tuners or race cars. For a person to be willing to sink his/her time and money into swapping a new engine into an old car they have to be a little "irrational" in the first place IMO. This hobby is a lot more about emotion and feeling and the excitement of driving an old car than spending the least amount of money to go fast. Like has been said if you want to save money by using an LS motor there are millions of old GM vehicles out there to put them in and a HUGE aftermarket to build the rest of the car to drive like a modern 'Vette. I just don't understand why you would bother to get a Mopar and then put a GM engine in it when you know at least 90% of either GM or Mopar fans would hate the idea. Yes it's 'your car' and you can do whatever you want with it but whether you like it or not unless you keep the car locked up in a garage and never show it to anybody, guess what you are gonna run into people who take one look at that LS in the engine bay of your Dart/Cuda/whatever and say "huh that car's been ruined..."
Muscle cars aren't about being rational in fact muscle car fans are probably the least rational car nuts out there! Myself included for sure...