Who said that life was either a thin soup or a lumpy gravy?
It sounds like you have a major mechanical problem secondary to the overheating, but you didn't need me to tell you that.
Surfing e-Bay, I found a place that will sell you a rebuilt 300E head for $795 without a core. This may help you in your repair. Let me know and I will send you the info.
Good luck and keep us posted.