Has anyone changed out the stock carburator on an OS .50SX and installed the 40E carb???

I'm converting my stock .50SX over to the -H for heli flight. I'm not really sure about which piece needs to come out or off. Let me explain:
When I remove the stock carb, there's a brass insert (sleeve ?)inside the engine where the carb fits. There is also what looks like a nylon sleeve on the outside of the 40E carb. Long story short - either the brass insert (sleeve) or the nylon sleeve on the 40E carb has got to go. The question is - which one needs to be removed???

Anyone out there know what I'm talking about and can help??? I would appreciate any feed back.