Jager,
The actions that I have used to convert to 45/70's are 4 Beretta model 409's with crossbolts, 2 Aya's ,2 Laurona's, an FN sidelock, and one Ruger 20 gauge Red Lable. All the others were 12 gauge frames except the FN and it was a 16 gauge. I would not bother with any that were damascus, they are too hard to match up the barrels to and tend to be weaker anyway. Not because they are damascus, but because they are usually older guns with weaker actions. I especially would stay away from any of the older hammer guns unless they have threaded in bushings behind the firing pins. Too many were made with screws that come in from the side and hold the pins by engaging a notch in the floating firing pin. These can get badly worn over the years and can blow out even with shotgun shell pressures.
I had a friend who tried to shoot a modern 410 shell in a short chambered hammer gun one time. It broke the tumbler in the lock but luckily the hammer remained on the tumbler. The floating firing pin cut a big gouge across the bridge of his nose leaving him with a nice ugly scar for the rest of his life. If the hammer had come off, it might well have killed him. I saw that gun after the accident and the hammer would rotate 360 degrees in either direction while still attached to the lockplate.

There are many good German guns for sale that don't cost an arm and a leg. Merkels, Sauers,and Simsons are all made in Suhl and even though they are"middle aged" guns, they have good steel in them. Look for specimens with Greener type crossbolts on them, (the round type). The square ones tend to crack at the corners. Some will have sideclips, and many have a cheekpiece on them, which I like because I think they make the conversion look more like a rifle. I also try and find one that has good sound wood on it so that it does not have to be re-stocked. Factory sling swivels are also nice to have as far as I'm concerned. Bushed firing pins and/or a vented breechface are also positives.

16 gauges would be best for a 45/70 but they are getting hard to find. 12 gauges can work out just fine as long as you thin out the barrels quickly so as to get their weight down up front.
Bob H.