THB,
If this is your first conversion gun I would forget about building your own monoblock unless you have lots of equiptment and machining experience. As far as barrels go, if you use the 12 gauge barrels for the monoblock your barrel diameter needs to be as large as the shotgun barrels so they can be joined at that point. You will want to contour them to about the thinnest profile that most barrel companies offer. Even that may be a little heavy. Good luck with your project. Bob