Smitty,
Welcome to the Nitro Express website, I think you will like it here. Lots of nice people on here. Your donor gun should be fine for building a slug gun, either in 12 or 20 gauge. I have some nice E.R. Shaw 12 gauge barrels for sale at $100.00 each. These will make up into fully rifled sleever barrels that are almost 24 inches long. I also have the top and bottom ribs to go along with these barrels, if you need them. I have 20 gauge Verney Carron bull barrel rifle blanks that are 30 inches long. They are $300.00 a pair plus shipping and will be large enough to sleeve into 12 gauge actions.
I have built slug guns on lesser actions such as the Berettas, a Browning BSS, Spanish Lauronas, etc., and have seen them built on L.C. Smiths, F.N.'s, Savages, and Stevens shotguns. Let me know if I can be of help to you on this project. If you want them, I can also send you diagrams on the best profiles for sleever barrels in 12 or 20 gauge. That will help you to reduce the weight on your new double to get it balanced properly and have it come in at around 7 pounds or less. Bob