Since RickL ain't here, I take over his verbose commentary for him:
Been there, done that. (coolant leak wise...)
Your heads would be more prone to a pin hole leak, more so than the block. As Rick P said, do an initial pressure test with the valve covers off to see if you've any tell tale internal drips. With the system pressurized, look down into the pushrod holes with a pen light to see if anything's detectable. Check as many of the head bolts' torque as possible while you're there. Your next step would be to pull the intake. Be careful when pulling the intake. You want to preserve the intake gasket as much as possible for inspection. Take a close look at both gaskets for signs of leakage/undue deformation and a uniform crush by the intake. If nothing makes itself known then I'd pull the heads. Take a look at the head gaskets, especially around all the water ports. Again, if nothing stands out then I'd send out the intake and heads for a pressure test. It that proves nothing then you can pressure test the block in place, right side, left side, individually.
It sounds like you're getting a relatively small quantity of coolant in you
oil, like I did. It wasn't enough to make the
oil milky, just enough to produce vapors and rust things up. You should still have had enough run time to build a bit of a milkshake in the valve covers though. take a look at them.
let us know as your troubleshooting progresses.