I didn't read most of the replies, got tired after the first page of reading diet advice. If you're thin, good for you. Keep the advice to yourself. Rant over.
As to the seat, I found the quick and dirty way to free up a few inches (until a better alternative arises) is to cut the upholstery off the seat back, then replace it with one of those wooden chair seat cushions. Heck, I think mine is taped on. I'm in the middle of bobbing my deuce, so the seat back is really low on the list.
On my first deuce, Psychotic Reaction, I used an old Hummer driver's seat frame modified to fit a springer base, and used a two part wooden seat cushion. Worked wonders through it's last 2500 mile trip.