Obvious, being a motor guy, you probably pulled the cover off right then. If it was overheated, believe me, you would smell it, feel the heat and probably still see a little smoke from the paint. I'm with Frank. I think you lost a piston. 70s were good motors, compared to the V engines, when it came to pistons; but they would still lunch one here and there. Especially if you're using the VRO system. Also, don't forget that lean running will fry up a piston. Maybe you had low fuel pressure for whatever reason or crud in the top carb.