I am not experienced with the 500s/550s but there was a post a while back discussing the difficulties of mounting up the Gl trees to the 500/550s head stock. As Duster mentions, it apparently is not a direct swap but rather it requires replacing the GL steering stem with a 500/550 one. This involves grinding off the welds, pressing out and then back in the stems and rewelding. The GL forks are also most certainly longer than the 500s. Good luck.