In theory if you bore the monobloc for the barrels so the barrels touch at the muzzles and the center of the bores are level you will be really close.
You want to explain that to me? They would be tapered inwards in relation to each other by a couple degrees just due to the outer profiles of the barrels. If each barrel is angled towards center just 1 degree (2 degrees included angle) the center line of the theoretical perfect pattern will be offset to the other side about 2 feet at 40 yards. The patterns might overlap at 40 yards, but the center of the patterns would be four feet apart. I would say, "if I did the math right," but I used a triangle calculator on my cell phone. If the ap did the math right.
My plan was to basically bore the block and make spacers such that the center line of the chamber was almost exactly the same distance apart as the center line of the muzzle.
P.S. I've been playing with the idea of using a clamshell design as opposed to a one piece mono block. It would simplify some of the setup issues for machining. I'll probably cut a mono block anyway, but I've got to setup and figure out the basic geometry and do some basic plans before I decide the best approach for me. The center line of the chambers is restricted by the firing pin locations on the frame.