Your best bet, if you are concerned about NFA and such, it to finish off your plan and then send a letter to the ATF accurately describing your plan and ask if it is OK. They will eventually reply with statements that explain the law and say yes or no or do it this other way and it's OK. Generally speaking, you're OK if it is a muzzle loader or does not use ammo with cartridge case, powder, primer and projectile as a self-contained unit.
But a letter is the most sure way.
I would suggest a shrink fitted reinforcing band (like a Parrott) around the chamber and an internally threaded plug with a couple of inches of threads. Then you could shrink on a trunnion band to hold the trunnions. There were many styles of guns used in the Civil War (though not in the quantity of the Napoleon, the Parrott and the wrought iron rifle) so it is likely you could find one the looks more or less like what you would build.
And you might find that you would be better off cutting the barrel into two pieces and building two guns because 89" long and 400 pounds is a pain to manuever.
