[ronstory]

If you have stuff at the bottom of the threaded holes in the block, I would soak them with PB Blaster (or similar)... and then tapping them with punch and hammer. Be careful of the threads and wrap a bit of tape around where they could contract the slides. You are just trying to break the threads lose and not drive them out.

Then do the drill thing, but the tapping may make the process less arduous. ;^)