I’m not sure this is a headspace problem. “I could not close the bolt completely”. Was this with an empty chamber or with a round in the chamber? If it closes on an empty chamber and not on a loaded one you have an ammunition problem. You check headspace by trying to close a healthy stripped bolt on a GO and a NO GO gage inserted in the chamber. This is not a healthy bolt. This bolt has been under a lot of duress. Some of the wear to the upper left lug is the result of using force to close the bolt somewhere along the way. My thinking is don’t spend your money on this bolt. Look for a better one. The Trader section of this Forum has plenty of them.
Why do you have to modify a K98k to fire blanks? Is it a feeding issue?