Well not just the insurance companies the NHRA had a bit to do with it. They up the quantity for SS to 500 and run Ford out of the game and later in 1967 or 1968 reduce it back to 50 under pressure from GM and Chrysler threatening to pull out too. Then Chrysler wins the '68, '69 and '70 SS world championships with a guy (I can't recall his name) that was campaigning a '64 Comet in the '64 season as I recall. He switched to Mopar because of the Ford pullout. This is as I remember so the exact years of the championships may me off a year. No doubt Chrysler was intending on putting their stamp on both drag racing and NASCAR back in those days.
It was nice to see the Gartlett interview on ESPN2. He was a big Hemi proponent. I think he was the first to break 7 sec., although they really didn't say that in the piece. Hasn't had a beard since!