Try a 0MT reamer, from the above chart link I posted, big end 0.3561 small end 0.2520 length of reamer 2". One of these will set you back about $30, maybe less if you look for one. I have one of these, I use it on one of my trumpets, and have also use it on some AR inserts to open up the exhaust end.
The 1MT size wise looks to be too large, but if you have the insert chucked up in a collet chuck, it would be nice for opening up the exhaust a little more and get a smooth transition.
I am interested in the tapered barrel, not for the Reelfoot, but for something else I am working on. Or should say I am wasting a lot of time on. All of the theories of the mouth blown calls go back to the duck calls, the goose, and the predator calls work on the same theories.
Marvin