Sorry Jeff,
the sensible answer is that it would not be a good idea to proceed in these circumstances. IMO there is not enough metal there to do what you want. Given all the money that you will end up putting into this project it is better to start from a firmer foundation.
However....there is a good chance that it will be perfectly OK. So if you want to proceed then I would do it on a dovetail lump, because if you make it up, test fire it, and it moves then you can have the joint heated again remove the dovetail and use the same barrels for a different action with a new dovetail. But if you turn the barrels for a monoblock for that action then they will fit nothing else and will be scrap if it dosent work.
That action will be no different from those that the Spanish use for their DR's it is just that they make the underlugs wider when they make a DR and the bolt corrispondingly wider. Sometimes they leave bolsters on the action sides streaching up to the fences when they chisel them up but as you have a 10g there then the metal is still there anyway.

Regards