before you go grinding and cutting...
do you use the gun in 303?
if so is the HS correct for the 303 barrel?
if you answer yes to these questions then modify the 7.62 barrel by facing off the breach and running the chamber deeper. there is no sense in modifying the existiing HS if its correct for one of your barrels

also from your pictures I cant tell if you are headspacing off the case or the bolt stops.