Although Dillon does not catalog an RL-550B caliber conversion kit targeted for the .450 Bushmaster, here's what you can do to setup your Dillon like I setup mine.
Obtain the following 3 parts and they combined represent a .450 Bushmaster caliber conversion kit (Dillon catalog item # in parenthesis):
1) 460 S&W powder funnel (#18949)
2) #1 shellplate (#13692)
3) #1 locator buttons (#13930)
My toolhead has Hornady dies installed. Only making use of 3 of the 4 dies provided by Hornady: the sizing die, the seating die, and the crimping die.