Roger,
I'm afraid that your gun is not strong enough to build any kind of rifle on. It has no double under lugs at all, the only lock up on it is the barrel extension and maybe one sliding bolt,and that is not enough to be using to build a rifle on. Do yourself a favor, if you are going to go to all the trouble and expense to build a double rifle, get a good European shotgun with three locking bolts on it and use it to build on. That way, when you get done, you will have something that is worth keeping.

Look for a gun with a Greener crossbolt on it if possible, or at least a gun with a hidden third fastener like the Purdy sliding bolt. It is always tempting to convert one of these old junkers since they are no good as a shotgun, but it is always a mistake because of their inherent weaknesses.
You can buy a good donor gun fairly cheaply if you find one that has bad barrels, that's the one to get and convert. Bob