pull your petcock off and measure the bung on the tank to make sure that it is the same size. Also, the 550's used a dual outlet petcock (not sure about the 500's). You can always put a "T" in the fuel line though and use a single outlet replacement one.
Partsnmore.com has a universal dual outlet petcock. It's a bit more money ($30 or so I think) and I'm also not sure of the thread size.
Tracy