What head gaskets did you install? I've had good luck with Fel-pro 9333PT-1 head gaskets, they seal-up really good... I've been through similar circumstances you are now experiencing. I didn't use RTV around the water ports on the intake gaskets... I do now
I've popped my fair share of head gaskets as well
Taking the heads to a machine shop to get them cleaned up isn't a bad idea at all... At the beginning of the year before I put my motor back together I had the heads milled to clean up the mating surface, they were off only .004" but every little bit counts... plus the freshly machined surface "grabs" onto the gasket better.
If coolant "poured" out of the #6 cylinder, I would say your head gasket is to blame... but you won't know for sure until you tear down the motor to the short block and check it all out.