Mike,
"Power" is usually estimated based on bullet weight and velosity or bullet kenetic energy but "power" wasn't the best choice of words. You're interested in how much stress the cylinder and frame can take and the best number for that is chamber pressure. Generally, a larger volume case can drive a given bullet faster than a smaller volume case at the same pressure or attain the same velosity at lower pressure.

There are factory loaded .41 magnums available that produce lower pressure than hunting or full power cartridges, but not necessarily low enough pressure to be advisable for your gun. I don't know what pressure limit is advisable or the pressure of down loaded factory cartridges but there is a good chance that cowboy loads are sold for .41 mag. they would have the least pressure. If it were mine, I'd have a worry free fun time firing home cast 200 to 220gr. bullets at about 800 fps. Cowboy loads are usually assembled to produce very low recoil and may have even lower pressure.

I like skillfully crafted old project guns that made sense in their day but make no sense when viewed from today's perspective. This one appears to have been skillfully crafted and I can understand why it would have been done in the early 1970s when used 29s readilly sold for 75% over msr and used 57s only fetched $50-$75 less, but they should have at least used a 28 or 27 frame. Maybe the owner who modified it did the conversion themselves using what they had on hand and knowing its strength limitation, enjoyed it for their lifetime firing low pressure reloads. That makes sense, but this gun makes no sense to me for a non-reloader.

Best Regards,
Gil