If the gun is pre-1898 and you can give some evidence of that (a statement by the seller, serial number reference etc), you won't need a form 6 or a receiving FFL/importer.
When I had a pre-1898 double come in by air freight, I had to go to the air freight office at the airport, fetch the incoming paperwork, take the papers to the local US Customs office, get their approval, then take the resulting Customs doc back to the air freight office where they then released the gun to me. It wasn't in MA, though. How this might all work out in Boston is a mystery.
If it is a post-1898, you will really need an importer or several months of ATF form 6 time.
