I'm a big guy and i am fairly certain I can fit in the 8181S. I sat in one with aluminum seats and seat rails and I fit, but the shifter and steering wheel were a problem. I figure without the seat rails, I'd get 3" back and have plenty of room. After building a GTM and struggling to get a seat that gave me the headroom i felt comfortable with, I want to be 100% sure before I start.