Ken,
Your concept sounds viable. The monoblock concept would save a bunch of handwork if the monoblock was machined/ground to fit the reciever breachface.

On a segment of Shotgun Journal, they showed Bruce watching a gunmaker fit a monoblock to a O/U prior to tube installation. I don't know if the tubes went into stepped holes with no barrel coming all the way to the breachface, or not. Something to think about. From structual standpoint, there's no reason to extend the tubes all the way to the breachface, especially if soldering is replaced with welding. The problems with not extending (stepped hole) the tubes all the way to the breachface include close tolerance of the monoblock hole depth and barrel steps along with a potential to get moisture/crud in any gap. On my sleeving job, I welded the breachface of the tubes to the monoblock.