The studs are machined on a taper to match the taper in the boss where they bolt up to provide an interference fit. Hold a big hammer on one side of the boss and smack the other side of the boss with another big hammer. This will usually free up the stud and it'll just drop out of the boss.
Just for explanation, the taper is different component to component and car to car and is based on "X" number of inches of taper per foot. ie: 1" per ft, 1 1/2" per ft., etc. Hot rodders who are unaware of this can mismatch components and create havoc.