I am 6'6" and bought a Mustang super wide touring seat. Comfy, but it moves you a head 2 inches easy. Have you looked into CycleVisions 2" floor board kit? It gives a little more room, I put one on. Pretty easy to do and only around $100 for the kit. Hot Bike or American Iron magazine just had an article about a seat in the last couple of months, that is supposed to move you back, but you have to move the computer.
I have the FLH version of the HD Tallboy seat and it gave me some precious inches. Very affordable and comfortable too. I do a lot of touring and can spend 12-16 hrs on the bike with mine. Looks like it is available only for limited models though
The most room is from a Lepera KingCobra, but it has almost no padding for a passenger. It will move you lower and farther back. Corbin also makes a version. Next best is the Corbin gambler, but about the same deal for the passenger. The best for you to try is a stock Fatboy seat and pillion. the pillion has a slant cut in it that allows you to slide back a few inches. The heritage seat with the nice pillion back support pushed me up into the tank, at least that's how I felt. Good luck, I'm 6'4".