As Joe said just close the gun smoothly, not slammed, & let the lever snap home, this is why they're called "Snap Action" & the way they were designed to be closed. The Ithaca built NS was Ithaca's first entry after the development of the new "Super X" shells in the early 20's & was designed for them. One properly cared for & used only occasionally with heavy loads as you state, will outlast you.
An option for the Crescent is to simply shim the hook until it closes tight. Various substances have been used for this purpose including feeler stock, aluminum can, even paper. It can be glued in place or simply stuck with a light coat of grease while gun is assembled. In this case it may fall out when gun is disambled & have to be put back, but will stay put as long as gun is together. If you reload you can find 1¼oz loads at less than 1200 fps, say around 1150, which with a proper size shot will work great & not put near as much stress on the gun as full blown Express loads in excess of 1300fps.
