I'd go so far as to say that there is probably no better time to own a mopar than now. Even compared to back in the day. There is more technology, more development, and more parts available for more combinations now than there ever has been in the past. This includes engiens, driveline, chassis, body and interior.
The reason why you see the price differences is simple supply and demand. Here is an example; for the entire production run from 1970 to 1974, there were fewer Cudas and Challengers built, combined, than any single year of Camaro or Mustang production. That number is also representative of overall production figures for all makes and all models for most years, so just by their nature mopars are a more scare animal, which means fewer cars are out there that need parts, hence less demand, which means development and manufacturing costs are spread over a smaller customer base.
However, the converse of that is that the parts we do have access to are usually more pertinent to bonfide performance than other makes. You won't have a choice of 15 different header bolts for a mopar, but there are any number of hard part producers that can make anything for a mopar that they can for a chevy or ford. So, while we have to buy new heads and camshafts, we may not need new cranks and rods, so total cost to produce comparable amounts of power to other brands is actually much closer than you might think.
Probably the biggest improvement, across the board, any size engine in any car you can make to any mopar is breathing capability. Heads are the real pinch point that can reap benefits when improved. Everything else is usually much more stout than it needs to be a for a street application. Even in racing applications, there is not a huge amount of changes that are required for high power and long life.
The cars are not much different in the repsect than the engines. They were very progressive for their time and had engineering in them that was decades ahead of others. Another example; 1st gen camaros and mustangs have some significant front suspension geometry issues that require a lot of work and/or parts to correct to make them great handling cars. Mopars have never had these types of issues and typically came with brakes, transmissions, and rear ends that were more than strong enough to put up with the heaps of street abuse that was laid on them. Ex #3; the brakes that came on first generation GTOs are the same size as what come standard on the 6 cylinder economy mopars.
Outside of that, you really need to get something in your hands before we can say what it needs to be better, faster, stronger. If it is a 65 Barracuda with a six banger, it is going to need more and different stuff than if you score a 71 Challenger big block 6 pak car.
BTW, welcome to the club.
