My husband is 6'4" 320lbs, and has the same issue as the OP. After 3 different seats, he finally found the perfect one. It's the "Tallboy" from Harley. It moves you back an 1 3/4, and up 2 inches. It's a super comfy ride too, and also a 2up. Hope this helps.
I did just realize though that the orignal poster is riding a sportster, and not sure if the Tallboy comes for the sportster. My bad! =)