There is supposed to be a bit of clearance between the tip of the rear tang and the wood - exactly created that crack, because the rear of the tang is acting like a recoil lug and transferring recoil to the stock there. When you have it apart - take a good look at the recoil lug recess - is it pounded back? Quick fix is to insert a shim - say .020 or .025 between the recoil lug on the action and it's wood recess. Best fix is to glass bed the action, but have a .025 or so shim around that rear tang to create that space that is needed on a mauser.