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Bill,
Welcome to the wonderful world of building doubles. As a general rule American shotguns are not a good choice for converting because they lack sufficient lockup mechanisms to do a good job. Many of them do not even have double underlugs with two bites in them, let alone any kind of third fastener such as a crossbolt or sliding Purdy bolt. There may be exceptions to the rule but generally the American guns are not strong enough in the lockup department.
What you should be looking for is a European mid to heavy weight gun with double underlugs and a third fastener with the Greener tapered round bolt being the best. Other things that are desireable are double triggers,(singles will work but are not as dependable),bushed firing pins, and sideclips. Sideclips may not be as important as the other items but are considered helpful in keeping the barrels aligned with the breech, and many european guns have them. The other thing they are good for is channeling the hot gases away from your face in the event of a ruptured primer.
Another good thing about using European doubles to convert is that they have proof marks on them that will tell you at what level of pressure they were proofed. This allows you to make up some proof loads of your own to test your conversion gun. Without some idea of the strength or your donor gun, you are just poking around in the dark. You will want to get your hands on as many reloading manuals as you can that actually list the pressure of their loads and their speeds. From this you can figure out a decent load for your new gun after figuring out how much back thrust pressure the frame can stand safely.
Other things that you might want to consider when looking for a donor gun are sling swivels and a good reuseable stock, with or without a cheek piece, whatever you prefer. If you do not wish to have to restock your gun, I would suggest that you pick a boxlock with the square back on it. Many of the ones with the scolloped backs have cracks started in the head of the stock. If it doesn't have one when you buy it, it may easily get one started when you begin to shoot it with rifle loads in it. At any rate, the square backs are much easier to restock. I deffinatly believe in bedding the new or old stock in bedding compound such as Acraglas. This not only helps to stop cracking at the head but it gives the gunstock close to 100 percent contact with the frame and help it to safely transfer the recoil energy to the stock.
One other thing that I have not seen mentioned on here is that you need to buy a gun with a big enough diameter at the front of what will become your monoblock. The monoblock is usually cut off at just a little bit in front of the barrel flats. Depending on what caliber you want to build and how big the barrels need to be in the chamber area, you need to size the gun frame to the needed diameter of the barrels. This is especially important if you intend to thread the barrel stubs and the monoblock. Buying a feather weight birdgun with 2 1/2 inch chambers in it, and expecting to convert it to any of the heavier calibers is as bad as trying to start with an American gun.
The gun's weight needs to match the expected recoil level, there are formulas to figure out what the free recoil will be in different weight guns. I hope this is of some help to you in picking out a good donor gun. If you can find one with these features and bad barrels, it will save you lots of money and there will be no need to cut up a good double. Bob