45-70 is pretty crazy, better get them on the first shot because the recoil is probably going to put the top of the frame through your forehead. I shoot 45-70 in a Marlin lever, cant imagine it in a revolver.
I've read that the .500 doesn't kick nearly as much as you would expect due to gun weight and clever gun design.
I have a handgunner mag that has a modified Vaquero that shoots .50, pretty sweet.