Chris, I applaud your enthusiasm!
Although I have yet to build a double of any kind, I do have some double rifles and have given a build a lot of thought myself.
One thing I may have missed in your posts is, what caliber/performance range cartridge are you wishing for and what do you intend to use your rifle for?
The reason I ask is that if you want to go big, you may want to start out with a 12 or 10ga with shortened barrels and see how it works with bullets, sabots or round balls. Some other members here have been quite successful in getting a smoothbore to shoot well and it could be just the experience you need as far as setting up a regulation jig and getting the barrels to shoot together.
Then, if that ends up not being your cup of tea, just cut the barrels back for a monoblock and go for the whole rebarrel job.