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Tarawa,
I think if you choose a robust enough 12 gauge, you can put your 12 gauge rifle tubes on it. The sleeves will be thin, but not overly so if the monoblock has enough meat in the walls to allow it.
They re-sleeve shotguns all the time back to the original gauges and the thin rifling in those tubes won't make enough difference to count. They will have to be fitted closely, of course, and properly soldered in with the right solder, but you shouldn't have any problems with doing them this way.
There are two ways to do the barrel stubs. The first is the way Beretta did it. Instead of boring straight through the monoblock, they stop the boring and reaming process about .150 to .200 short of going through to the breech end of the monoblock, leaving a reinforement ring of steel, (for lack of a better term). It extends at least .065 into the chamber area and is part of the monoblock. The shotgun rim cuts are cut into the back side of this reinforcement ring. The barrel stubs are turned down till the wall thickness is .065 after short chambering and turning the stub ends down. The forward shoulder of the barrel butts up against the front of the monoblock and the back end of the stub simultaneosly butts up against the inside shoulder of the reinforcement ring. The distance is critical so that both surfaces bump up at the same time. This leaves a hidden joint at the back of the gun's breech, in order to see the joint, you have to look very closely inside the chamber and down about .200 from the back. Most people don't even realise that their gun was built this way.
The other way, if you have enough barrel thickness at the front of the chambers, is to bore straight through and ream the monoblock accordigly. Your barrel sleeve stubs can still be only .065 wall thickness if necessary, but you will then be cutting the shotgun rims into the backs of the barrel stubs. If you have enough meat in the mono to allow it, ream the bores of the monoblock till you get rid on the old rim cuts, that does away with fitting and soldering in rings as fillers. Your stub ends will then be thicker and you don't need much wall thickness up front for the barrel shoulders to bump up on so don't worry too much about that. That's not near as neat as the first way, but it will work and not be too visable if the stubs are carefully fitted.
In neather case do the barrel stubs have to be threaded. The main thing is to keep everything centered well while doing the drilling and reaming. Bob H.