Have you considered having it threaded for choke tubes? One of the best shotgun barrel guys in the USA is Mike Orlen in Amherst. He'll tell you if it can be threaded.
If you don't want to change the original look of the gun, there are a couple of Ithaca forums that may be helpful - try Google.
AFAIK, the original Ithaca company changed the machining at some point in the '60's so that their barrels became interchangeable. If you want to test that out, PM me. I have a 20 gauge from 1969 with a Modified barrel, and we can see what fits what. I'm also in western MA. And no, you can't have my barrel.