Yes, you are inviting disaster. Your '66 is a toggle link rifle and they are not designed for modern high powered loads. The original '66 design fired a relatively weak 44 rimfire cartridge. Even with modern manufacturing, and modern metalurgy, the '66 still has a brass frame. It is the brass frame that will take the brunt of the backwards thrust of recoil from your cartridge. Read Mike Venturino's 'Shooting Lever Guns of the Old West' sometime. Somewhere in there he documents a brass framed Henry that was ruined by a few rounds of high powered 44-40 loads. Your '66 is basically the same as the brass framed Henry, design wise and strength wise. As scattered thumbs said, I would not even run rounds like that through my steel framed '73. The design is just not up to that kind of punishment. You would be much better off running rounds like that through a '92. They are a totally different design, and are as strong as a bank vault.