[notfarnow]

I'm not familiar with chev v6's, but the symptoms you describe sure sound like a headgasket or cracked head.

Works fine until the cooling system is overpressurized, then flow is impeded (no flow to heater core, lower hose cool, etc) and then overheats.

Exact scenario in my Samurai this spring, was just the gasket... head wasn't even warped. Same thing in my friend's Honda this summer.

Does it overheat if you leave the rad cap off?