Yup, the fiberglass backbreakers mount low, and some have quite a lot of lean to them. Any real seat will be much taller.
I have APC Racing seats in my buggy. They are mounted about 2" below the frame and have a lot of lean to them. At 6'1", I barely fit, but at 6'0" my father has a lot of room.
My frame was built by XTC with the cage raised 1.5" over stock.
Can you post up a side-profile of the competed car?