The hinge and the latch piece are both bolted to the seat pan. Just unbolt the bracket and latch piece and set the seat in place to check fit. I think you'll notice the underside of the 500F seat is not a real good fit to the 350F frame, and the rear rubber bumper misses the frame loop by about an inch, height-wise. The front rubber bumpers on the 500F pan are intended to sit on the round frame rails and are spaced about 3/4 inch too wide for the 350F frame, etc., etc. The 500F uses a different latch design than the 350F. The distance between the hinge pins and latch piece are about an inch wider on the 500F than the on the 350F. I don't think the upholstery swap will work very well, since the 500F seat is about 2" longer than a 350F seat. Good luck.