Rob,
One thing that you need to know is that the shotgun barrels converge from the breech face to the muzzles, therefore the chambers are not parallel to one another. You would do better to set up the monoblock in a vertical mill, indicate on the inside walls until square, bore out to your thread's major diameter, thread with the appropriate tap, and face off square to the centre. That's how all of my conversions have been done. I always DO thread AND solder, gives a 100% strong union and provides a crush fit between the barrel and the monoblock so that the seam is barely visible. You want to use a tap with the maximum # of threads per inch in order to keep the thread depth to a minimum. On the one 10 gauge that I used, 15/16"-32 worked well.