Does the 550EX have a sync socket on the side, like a lot of the Nikon flash units? If it does, pick up a female-female sync cord like the Nikon SC-15 and use it to connect the 550 to the FTn directly. If not, then use the adapter. I would personally use the bracket, since the accessory shoe that Nikon provided for the early 'mats seems to need an eyecup (Nikon DK-4, btw) on the eyepiece ring to help keep it from rotating when a flash is mounted.

-J