Slant six over heated and now will not keep an idle.

Is your temperature gauge working? What was it reading? What was your Overheat symptom. Perhaps it was a heat soak condition, I had the same issue in my car when my slant was a daily driver. I'd go to work, and then in the afternoon the car would have a REAL hard time starting up. Nowadays I blame it on the ethnanol gas that boils like a tea kettle in our carburetors. Check out the Fuel Line Mod Here: Slant Six Forum, :: View topic - Fuel Line Mod Improves Starting & Driveability

This may help your hot start issue. Otherwise if you think its a warped head, do a compression test for each cylinder. A classic sign of a warped head is burning coolant or air bubbles in your radiator.