MP and bag for monsters

[Gorilla Tuba]

This is a backwards answer to your question. When I purchased a bag for my Conn 3J, I was advised to get one designed for a Besson EEb. So, to answer your question... it should fit.

Your mouthpiece question: I have a little Holton EEb. It came with essentially a big ol' bass trombone mouthpiece. For me, a fairly standard tuba mouthpiece works fine. Specifically, I have a marcinkewicz W1 (which is shallow and wide) that I lathed down to fit the small reciever.