Slant won't start!!?!!?!?

Are you sure that your compression gage works and that you can seat it fully in the deep recess of a slant? Did you pull out the spark plug tubes to get a good seal? Do you feel good pressure with your finger? For comparison, home water pressure is ~60 psi (finger over hose feel). I can't imagine how a failed head gasket or crack could take out all 6 cylinders, so suspect your technique first. Sounds like the engine ran fine before. Maybe invest $15 in a Harbor Freight screw-in compression test kit. Slant heads have little problems with cracks that I have read about, nothing like some engines, and boiling over doesn't usually hurt the head.