I have a question about mixing of parts from various car models to make your own model utilizing the best stuff from each of the factory models. I thought that you couldn't do that.

For instance, here is a post that is a good example: (Sorry for using your post here, anonymous person, but it was exactly what I was looking for.)

______
A few years ago, when I was running an all-S5 ('89) car, the concensus I was hearing was that the S4 ECU and AFM were superior to the S5 but the S5 intake and rotors were superior - so the hot setup was an early car w/ late engine. At the advice of one of the Fla. shops I went so far as to try to retrofit an early ECU and AFM into my '89 car.
_______

? I MUST be wrong on this, otherwise everyone wouldn't be doing it. But what exactly is the reasoning? I understand the rule is that you have to pick a car model, and that's what your car is. So you can't use parts from other models. Like you couldn't use a 5th gear from a GTUs unless your car was a GTUs. (Or you made your car into a GTUs.) That gear ratio only came in those cars. And if your car was a GTUs, then it didn't come with an early body, wiring harness, etc. Enquiring minds want to know!