As I said, the books say "PowerGlide" exclusively, yet I know of at least 2 '66s with TH400 (both big block cars). I don't know the production dates of those cars. One of them, I know for sure, was original. The other I can't say.
There is no doubt GM did NOT put a TH350 in there, as '69 was the first year of production for TH350.