I have had some success in the past removing distributor cam screws as follows:
- Drive the pin out of the collar at the bottom of the distributor shaft and remove the collar to expose the tab at the end of the shaft.
- Get out your set of hollow-ground flathead driver hex bits and find the one that best fits the cam screw.
- Put a magnetic hex bit holder socket on a socket wrench with a nice long handle and put the flathead driver in the socket.
- Clamp the tab in a vise.
- Pressing down on the head of the wrench with your left hand to keep the bit from camming out, release the screw using the leverage provided by the wrench.