Rarity (and desirability) of a slant six/ factory 4 speed?

"That said... "rare" means nobody wanted them when they were new, either!"

True, but sometimes things are not popular when new later become very sought after. That's because hindsight can be 20/20, or because the world changes. An option seen as frivolous in 1955 or 1964 might later be recognized as 'awesome'. Not many people bought EFI Chrysler 300's in 1958, and the ones that did had them break down....but if anyone had one today it would be highly collectible. Probably one of the cool aspects of a Slant/4sp is that someone had to go out of their way back then to order the car as such....and of course it's one more gear.