I'm not at all sure that there is a drop in shotgun Bbl for the T-C Hawken. The New Englander was built as a combination, though, it had round barrels.
Three Hawks
I "think" 3 Hawks is right, although things may have changed since I last dabbled in this possibility.
The barrels you are looking for are all full in 1 inch tubes. The .54 hawken barrel is 15/16ths across the flats and that puts the lock out-of-whack trying to install it on that platform.
Now, having said all that, I will say that one of the guys here, and I can't remember who, said he had managed to work around the problem and could do it.
I can't remember who it was, and I never saw the completed rifle.
And, after talking to Dick at Pecotinica Longrifles, I personally gave up on the idea.
Maybe he will come forward and help out a bit here....I believe it was Tom, but my ole rememberer just ain't up to snuff any more....seems it keeps getting over ridden by my forgetter.
Uncle Russ...