If the trunion DOES look like a 7.62x39 trunion, then you can reasonably expect to use a 7.62x39 parts kit and build from that.
A thought - on the books it would still be a shotgun, correct? If so, then one should consider the need for an 18" barrel no matter the caliber! Otherwise it would be *classified* as a short-barreled shotgun! *oops*
I wonder if thera are any special requirements for "reclassifying" a shotgun as a rifle? Those huge builds that Hubel is working on are all technically shotguns, and as such are not classified as Destructive Devices. I'd be concerned about using the shotgun platform as a base for several reasons.