You might be confusing the fact that bass trombone shanks are very close to the old small Eb tuba shanks. However... there is a vast difference in the size of the mouthpiece cup.
Dan Schultz
"The Village Tinker" http://www.thevillagetinker.com" target="_blank
Current 'stable'... Rudolf Meinl 5/4, Marzan (by Willson) euph, King 2341, Alphorn, and other strange stuff.
What Tubatinker said. The old eefer shank, which is also about the same as the old British shank, is very, very similar to a stock bass bone mouthpiece shank. (Personal experience)
Because of the difference in range and geometry of the horn, the cup, throat and backbore of a bass bone verses a tuba mouthpiece are different to get the respective tonalities of bass bone and tuba.
That said, if you want to experiment with a little larger mouthpiece, Denis Wick still makes the small shank as a stock item, and a Wick 4 or 5, as a small tuba mouthpiece, is comparable to some of the larger bass bone mouthpieces.
Some guys, for some repertoire, do use a tuba mouthpiece to get a different tonality on purpose. There are discussion threads on this topic on this board.