I was thinking of taking a long chisel or screw driver and beating on the end of it to knock the rest of the screw sideways and (hopefully) loose, but that probably won't work. I'll try the hacksaw or a really big set of bolt cutters and see if I can get it.
This is a 1964 Mercury Comet Cyclone with a 289-4V. I am still a tiny bit optimistic that we can make it run...the engine still turns, but my hopes are fading the deeper into this I am getting.