I have a Ruger Blackhawk with an alloy grip frame. I just bought a steel grip frame from a forum member here that was for an Old Army. I thought they were interchangeable, but when I got the grip frame, I see it's almost the same, but is missing a pin that a spring mounts on. It looks like it's simply a matter of drilling the hole in the new grip frame, and taking the pin from the old one, and putting it in the new one. Am I missing anything? If that's all that's involved, how much should a gunsmith charge to do that?