My monoblock is based off of the 12ga set from the factory, caliber is going to be .500 ne or more likely .450 3-1/4" because of the cheaper and more wide availability of components. Is there any negative aspects to using even finer threads than the 7/8-20 as Brown suggested? Looking in my reference books I do see specs for 7/8-24, but with each increase in thread pitch there is also the need to ream a slightly larger hole but I guess it really goes by the starting thickness of the 12ga chamber walls.
I would be concerned with warping if the barrels have to be heated along with the monoblock. I'm not familiar with heat fitting but I do know some materials have different rates of thermal expansion which would have to be taken into account, or at least I think they would.