it is a "stretched tank" for the 550. to put a 750f tank on my 500K1, I had to shorten the posts on the frame as had been mentioned, and then cue my stock seat pan about 2" in the front to allow for the longer tank. I came up with a mounting system for the rear of the tank to keep it secure. there is a pic of my cafe project in a recent link titled body pics. that post has a pic of the longer tank on my bike.
hym