Been communicating with Ames. They did some tests and acknowledge that the color of the paint they have in stock for 75 to early 77 Pontiac V8s is really dark. They're acquiring samples from a different supplier to find out if that'll be a better match, then they'll replace the paint they sent me.
Meanwhile, I acquired a can of paint from Hirsch Auto. The difference is dramatic:
THAT'S the color I'm looking for on the right.