I don't know... my Euro shank mouthpieces fit in the bits of my Conn sousaphone just fine. The question should be "is the insertion distance on a sousaphone bit as critical as it might be on the mouthpiece receiver of a concert tuba?" Most bits are the same size and they just insert into eachother while others, like the King bits, are different sizes and are meant to be inserted in a specific order.
I know.... I didn't answer anything... I just made it a bigger mess by asking more questions...
What bloke is trying to say, and I agree with him: it doesn't matter.
That said, King souzy bits are stepped. The one that goes into the gooseneck is marginally larger than the one that the mouthpiece fits into. So if you get a pair of King #2 souzy bits, (arbitrarily calling #1 the bit the mouthpiece goes into) you should be fine.
I suppose a person COULD use a sousa bit to create one of these:
Mind the gap.
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.