How important is it to be able to get under that seat quickly ?
Depends on what you've got under there.
Seriously, if you have all your electronics and what not under the seat, you might want them to be accesible on the roadside in case of a break down (or blown fuse or what-not). Having to haul out a tool kit just change a fuse is a pain in the rear.
As far as mounting the seat, the frame rails are pretty much the same between the F's and K's. The only real difference is that the F's had that bigg 'ole tool box in the back where the K's justr had a fender. You can just put a K fender on an F, or a the F tool box on the K.
For my F1 that I cafe'd out, I built up a "cafe" seat out of the stock seat pan. basically just molded a fiberglass hump over the back section and cut down the foam on the front. Came out lookin't pretty cool. I'll try to think to post a picture of it when I get home.
