I fell the need to pass on some positive vibes that I have been experiencing lately as head gaskets go.
The last three vans that I have been working on, primarily testing and parting out, all blew bubbles into the overflow tank once they reached operating temperature.  After pressurizing the coolant system, doing a leak down and compression test on all three I received differing numbers.  The coolant system test seemed to be what was the constant on the failing side.  Well, long story short, the rear heater valve and/or the heater core itself were leaking badly in 2 out of the three.
After disconnecting the rear core and looping the hoses, a quick bleed of the system and no more bubbles!
These vans are not registered and don't see prolonged highway speeds.  Just wanted to pass this on.

The next time you think the head gaskets are shot, check the rear heater core and cross your fingers.

dylan