A monoblock would need to place the barrels further apart only if it is a significantly weaker joint than a chopper. IMO, the two piece nature of the monoblock breech is equivalent of the single piece nature of the chopper; it does not need to be thicker to compensate for weakness in the joint.

A "shoe" is any of a number of plate designs that cradle the barrel breeches and and join them vis soldering/brazing. The shoe also contains the hook and lumps. Look at the drawing on Hollowell's.