i have a 1972 buick skylark with a ignition lock cylinder broken off in the case

Either you have to remove the whole column or cut the column with a Dremmel tool and re-weld it later.

I don't know how much damage you have done to the steering column. If you haven't already messed up the lock cylinder housing, you can drill the old lock cylinder out. start with a 7/16" or 1/2" drill bit, drill through center of the lock cylinder. Check often to be sure you don't drill past the length of the lock cylinder.then use a flat blade screwdriver to pry the remaining wall of the lock cylinder towards the center of the hole you drilled. Pull the pieces out with needle nose pliers, you should be able to see a slot in the bottom of the hole when finished. You can use your flat blade screw driver to turn it clockwise to the on position, where you will be able to install the new lock cylinder.