MacNaughton:
I think you are on the right track with your choice of the BP .45-70 chambering, any higher pressure could be putting this old gun at risk. Others here may be more cavalier than I...

BTW, rejointing by tig-welding the hook and/or replacing the hinge-pin to bring the barrels back on the face will normally tighten the bolting naturally without the need for any additional tig work there. Try it first before putting a welder anywhere near the bolt (that 'ladder-shaped piece'). The position of the top-lever will tell you whether the bolt or bites need additional work.

...and if that gun were mine, I'd simply tighten it and assemble a bunch of BP bismuth loads for fun-days on waterfowl.