If the frame doesn't fit you, get rid of it. You could spend a lot of time and money trying to make it better. But, the bottom line is, the frame itself doesn't fit you.
I use a Corbeau Clubman seat. It's easily an 8 hour seat. Very comfortablt, very supportive. I take long drives in this car, and don't have any real complaints about the seats. And I'm a little bigger than you are - 5'9", 200#.
http://www.racing-seats-usa.com/20221.html