The Chapparals were banned because others could not duplicate Jim Hall's innovations and remain safe. The car with two fans and ground effects shrouds brought on cheap copies that had failures. The car that had an adjustable wing brought on copies that had catastrophic wing failures.
The Ford Thunderbird was allowed IN because Ford didn't have anything that WAS legal. Take a 1984 Monte Carlo SS (fit the rules), and park it between a 1984 Thunderbird and a 1984 Camaro, then see which two cars are the same approximate size and which one is a good bit larger. That was the end of NASCAR having bodies that lookied like real cars. After that we got what we have now, amorphous blobs that cannot be recognized outside of having the name brand on the front air dam.
The Chrysler Hemi ran NASCAR for years. The Ford SOHC was never allowed, the Chrysler (Chrysler built some prototype SOHC and DOHC Hemi based engines) answer was never fully developed, and the Ford Shotgun Hemi was allowed briefly. The Chevy 427 "mystery engine" wasn't banned, Chevy withdrew it because it wasn't ready, it was replaced with the Mark IV Rat motor.
One big reason the Mercedes Ilmore Indy engine was banned was because Roger Penske used Chevy money and development time and data to develop it, he basically stole from GM and gave it to Mercedes. That's why the great Roger had to leave Pontiac so suddenly and go to Ford. Smokey Yunick told the head of GM racing that the next time he (the head of GM racing) wanted to deal with Penske to let him know, and Smokey would send him a case of Vaseline.