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Ideally... what is the diameter of the small end, the flare, and overall length need to be?

Why not make a MTS extension to accompany the standard 1241 upright bell? Would probably cost a few hundred less.

5.25" would be near the base of most 4/4 bells, several inches up the stack. Most any should give you 20-24 in cut at that point.

well the standard 1241 bell is 6.25" tenon not 5.25". 5.25" would be somewhere in middle of bell stack. So if the bell is 12" and the 5.25" cut is 10" down the bell stack, that would be 22".