",,,nobody would run the powerglide..."
You probably already know this. But, since you are lookin for some hard-to-find racing details, you may wanna keep it in mind for future posts.
The '64-'66 GTO's did NOT run a Powerglide.
It was a BOP 2-speed. I think Olds called it a "Jetaway", Buick called it a "Super Turbine 300", & I don't know that Pontiac even had a name for it. They could use a TH350/TH400 type converter. I think there were some switch-pitch converter models, also. I realize that some people call all GM 2-speeds Powerglides.
https://en.wikipedia.org/wiki/Super_Turbine_300
Pontiac DID use a dual-pattern PG in, I think it was '70 & '71. I bought a Bird with one in it, from a U-Pull-It yard. Also saw one in a Lemans.