I have a Bally SS that the headback box key was lost. I have extra  keys that go in but don't turn.
Any suggestions on how to open or pick the lock.

I know that drilling out the lock is one way. What's the best procedure to drilling .

Any suggestions would be appreciated

Driilling> just a bit larger than the hole till it eats all the tumblers and turns freely. I had one that actually went thru to the other side once wouldnt turn but cam droped off.

If someone makes you a rocker pick out of a couple of blanks it is the safest way to do it yourself. Try finding some keys that are on the same keyway and move them in and out 1/16 of an inch while putting light pressure in the direction of unlocking and giggling a bit. It's a wafer lock and is a pretty un-secure.

If that fails you can always sell me your Scared Stiff.