If the barrel sets are in the $1500 range, then this company may provide a viable alternative. They can produce a barrel set milled completly out of a single piece of tool steel. I believe it was 1040, but the type of steel is not on their site anymore. They produced some o/u barrels for the John Wilkes company a few years ago. I talked to them in 2004 and o/u barrel sets in quantity were in the $1000 range. They would need a 3D model of the barrel for a quote. They should be able to keep tolerance on the barrel sets to 0.0005 - 0.001 for the fitting surfaces. This would probably allow all barrels to be exchanged with all receivers and eliminate most if not all hand fitting.
http://www.alphamachining.com/products_shotgun.html

I am willing to build the 3D model if you need someone to do that for you. I was a mechanical engineer for 10 years, so the model wouldn't be difficult for me, if I can get a barrel set to measure. I have access to ProEngineer and Catia solid model programs.

Let me know if I can help.