If it was me, I'd seriously consider buying a breeched barrel from TOW and going from there on my own. Not a difficult thing to do, the roadblocks are located between one's ears.
I have a T-C Seneca that came to me with a bad .45 cal. barrel so I had it reamed to .50 smooth and am now finishing up a .40 cal. Green Mountain barrel I had breeched by TOW. The rest of the parts I eaither made myself or bought. I made up the ramrod pipes and sights myself, not hard to do, and the under rib came from my junk box. They're available from almost all the on line parts vendors.
Just a thought,
Three Hawks