Brian,
Do you feel this donor gun is strong enough in it's lockup system to be building a rifle on it? I have looked them over before and didn't really care for them as donor guns. I guess if you were going to do a conversion on one, a low pressured 45-70 load would come as close as any to being a good choice. I recommended to him that he think about doing a 12 ga. rifled slug gun on his Marlin.
Another gun I hear about all the time as a possible donor gun is the L.C. Smith shotgun. The later ones would have better steel in them than the early ones but they still only have the one bolt in the barrel extension. I do not consider them a good choice either. That being said, I have seen a couple of 45-70 conversions done on L.C. Smiths that have held up so far. But I have to wonder if they are not over stressing that one bolt. A couple of other American gun makers also stole this idea from L.C.and used it on some of their guns, but I do not consider them as a good candidates either.
One of these conversions was done to help out a friend of the "gunsmith" who did the work. He made full length barrel liners that were somewhat loose at the muzzles and made threaded collars that were made eccentric to the outer threaded surface. By twisting and tweaking those threaded collars, they did manage to get it roughly regulated but I do not remember what the load was for it.
I also remember that he had to weld "ears" on the barrel liners back at the breech and machine out slots for those ears to fit into the 12 gauge rim areas because the barrels kept unscrewing themselves during the initial attempts at getting it regulated. I have not heard of this gun blowing up yet, and I think I would have if it had blown. Then again, I don't know the load nor how often they have fired it.
The gunsmith in question is an excellent machinist and he had matched the outside profile of the liner barrels to the 12 gauge chamber areas and some way down the inside of the 12 gauge barrels. It is possible that some strength was gained by this method but I can't believe it would be much. Also it did nothing to strengthen the lockup on the gun itself.
Lots of people out there would love to find a proper donor gun "Made in America". I have looked a bunch myself, and the only ones to surface so far have been the Winchester 21 and the Browning BSS, which I think was actually made overseas. There may be a couple more but all of them are marginal except these two and they are both expensive guns to be using for conversions. I think it is best, especially for beginners, to start with a good European made double with some kind of third fastener.
They can be bought reasonably cheaply and if you wreck it trying to do a conversion on it, you have not lost a couple of grand on a donor gun. Bob