The worst floater at high speed that I remember was a ‘66 Ford Galaxie with a 352 four-barrel.
Bingo! I had that exact car in highschool and I can tell you that somewhere between 90 and 100 mph that car got SCARY light. A good thing about the model was the front seat slid back clear to the rear bench giving immense room in the front seat for um drive-in activities. Steering wheel was completely out of the way.
My dad bought my mom a ‘69 Firebird 400. It was a beautiful car but got very flighty at speeds above 90. My mom was a lead-foot and that car was genuinely scary!