Seat width is irrelevant if the seat isn't comfortable, so personally I'd find something comfy then worry about making it fit rather than the other way round: An uncomfortable seat is just going to make you hate the car. Obvious I know, but speaking from experience of getting your heart set on something and then being disappointed when it's not comfy!
In terms of mounting, you'll really want to go custom plates and then bolt the side mounts directly to that. Rails will likely take you far too high. Literally you just want a bit of steel that sits flat between the stock bolt holes and then drill it wherever you need it.
6pt is the way to go, easy to fit but you'll need to get a couple of eyelets welded to the floor or you could drill it and mount through. Welding by far the best option though.