Unless you find a VERY thin seat go over the top. But the raiser is about 6-7 inches tall at the front. Most every seat is designed to sit flat on the floor. HOWEVER most later model seats are a lot thinner because the bodies of the cars go thinner. So a bench seat from a 67 Chevelle or something like that may work being they were MUCH thinner than a seat from a 1950 car or truck.
My truck had a chopped top, so that changes things a little. But I am not a tall guy and when I put 65 GTO bucket seats in my truck I removed the raiser and they worked PERFECT for me.