I have said this before and I'll say it again, the Thompson Center New Englander ( although not traditional in design ) is one of the very best black powder hunting guns you can get.
It is light, points life a shotgun, is quick to handle. If you get a shot gun barrel for it, you have the best combination you can buy.
One weak spot on these guns, if they have a wood stock, is that they often crack at the back of the barrel to the lock bolt.
Glass bed the stock and this will not happen.
It would be a shame to sink the extra money into redoing the barrel, and later have the stock crack. Bummer!
A .54 cal. barrel is a great way to go. Lots of thump.
That gun might be a little brutal in .58 cal. as it is so light in weight.
The flat butt plate will absorb a lot of recoil.
Fred