Only going off my experience with later models, and not directly with a '66:
Anybody know if trans cover plate was bright metal?
I've seen them in rusty bare metal, but even when mine was only 5 years old it was already looking mostly black with rusty spots. Not really oily black, so much as just "dark" colored. But doubt it was painted. Probably just bare metal without a bright finish such as zinc or whatnot, showing it's age.
Were all frame attachments (other than shocks) black?
You mean things attached to the frame? Or the frame bracket/mounting itself?
Either way, much of it was bare metal with a minimum of black paint, a bit of grime and a bit of surface rust. But yes, other than the chunk/third member on some being "rustoleum red" (I forgot for the moment what that color is known as) there was very little color under a Bronco. And even that red third member was a rarity more than it was common. Maybe different for the '66 again, but probably just as haphazard.
Wish we could have seen underneath Mr Kimball's '66 on Green Acres!
Assume gas tank galvanized?
Maybe, but never saw a shiny one visible in all these years. Always black or natural with crud.
Nothing authoritative about all that of course. Just some recollections to add my two cents. Hopefully someone that's done more restoring will chime in. Andrew at BajaBroncos, Nick from Nick's Trix, and Englewood cowboy do more of that. Not sure how long Englewood's been doing it, but know Andrew has been researching them for years. And around them even longer!
Good luck
Paul