My old 66 /6 Valiant had a drop down centre link already in it. That is how it came as I doubt anyone would change out that part. I also had another guy with a 66 /6 car that came with a drop centre link so I would check what you have before trying to obtaining one.
Yeah, I think the '66s more commonly used the dropped centerlink across the board- it seems '65 and back you run into the slant 6 links more often; but it still needs to be mentioned... :)