You indicated the bore was .343-.352. I don't know the exact size, that's why I suggested a diameter larger than the .343, a bit smaller than the .352. A .360 might be OK in .352, might be too much of a good thing in .343. As long as the ball is caught between the breech and the barrel, and doesn't freely enter the barrel, it should be OK.
If you are able to slightly ream the bore, you will be able to have good control over oversized ball diameter.
Turns out the nipple I needed had a finer thread than the -28. Made one on the lathe this afternoon, fits well, looks good. Bit of a sledgehammer driving a tack, using a 1600 pound lathe to make a percussion nipple.