Dave
I'd suspect perhaps cylinders 4 and 6, they usually suffer coolant leaks from the exhaust studs/ bolts.,,,,plus you have headers, and when antifreeze/water collect in the bottom of the tubes, it takes a while to burn off,....I bet your exhaust on the passenger side was fogging like a misquito fog sprayer!......see if there's still coolant in the header tubes, if so, disconnect the collector, drain the header, use studs if you can for the ports of cyl.4 and 6, use permatex gold silicone (hi temp), assemble, let dry before adding coolant,....I've had this problem before on header equipped cars, and the 1st fear was always head gaskets, cracks, etc,......use new gaskets when you put everything back together!
