I was actually installing the rubber umbrella type valve guide seals to stop oil leaking down the valve guides. In order to install the valve seals, the engine should be rotated until both valves close on the cylinder and then air is pumped into the cylinder using a leak down adapter in place of the spark plug. This prevents the valves falling into the cylinder when removing the valve springs to insert the rubber seals ocer the valve stems. I just happened to hear the gurging noise which evetually started pushing water out of the radiator. So I just lucked out on my discovery of the stress crack on cyl 6.
As a side note, I had a similiar problem with a 1966 Cougar on a trip to Florida a long time ago. When cruising the streets of Ft. Lauderdale during spring barke, the Cougar started overheating. I took the car to a radiator shop and they found carbon monoxide in the coolant. In that case it turn out to be a blown head gasket. Here is a link that explains the procedure for checking for head/head gasket issues.
http://repairpal.com/how-to-test-for-head-gasket-failureBudJ
