Exactly what makes a MOPAR a "Matching Numbers" car?

Agree !! Another can of worms!

I'm betting most of the regulations regarding powertrain swaps are emissions related. The rest is just added features and options.

The only thing you can't swap car to car is the VIN# . Its totally illegal!

(There... I just opened a fresh "can of worms" LOL !!)
You need to specify what brand and type of car. GM and Ford people are not so anal on the numbers crap. If that is your game, great! But don’t try to force everyone else to follow what you believe. I have worked in the GM Ford and Chrysler plants as a millwright and they use parts to keep the line going and at the end of the lines they have a manual tag machine and if the cars numbers have been screwed up that a new tag or number won’t fix they are taken off the sheet and either to the body shop or scrapped. These were never intended to be anything but machines until they were collectible. They are either worth $500.00 scrap or $500000.00 depending on the buyer and seller