The GTO was a beefed up mid-sized LeMans. Ford’s version was the Fairlane GT. The 1966 GTO had a 389 motor and the Fairlane GT had a 390. I had a ‘66 GTO convertible with a four barrel 389 and two speed automatic shifted on the column. A high school classmate drove her father’s Fairlane GT convertible with three speed automatic. My GTO was quicker up to 100 and she wouldn’t push her Ford any faster than that. The Galaxie was a heavy piece of iron like the full size Pontiac Ventura and Bonneville.
Years ago my neighbor had a 66-67 Pontiac Bonneville with a 421 but for some reason what really struck me about the car was the Kelsey Hayes magnesium rims on it.