The Merkel is an excellent choice for a donor gun, stong and lightweight for it's strength. I have shot alot of 400 grain cast bullets and also 300 Hollow points for hunting and some 350 grain flat points in my 45/70's. For hunting in North America you can't go wrong with any of them as long as you are not going for big bears. I load the 300 grain bullets and the 350's to 1500 to 1600 feet per second. This is a milder load than you said you were going to use but they do a good job on game and don't punish your shoulder too bad. You won't notice the recoil when you are hunting but shooting 60 to 80 rounds from a bench during regulation can soften up your shoulder over time.
It may not be proper to some peoples way of thinking but I put Pachmyr or Limbsaver recoil pads on all my rifles and shotguns. It really helps on the recoil and I appreciate that since I dis-located my shoulder deer hunting last year from a fall on the ice. Good luck with your project, you won't be sorry. Bob H.