[Michael C Henry]

The temp sending unit is in the front of the intake manifold.If the whole system isn"t circulating emagine what tempreatures that the rear cylinders and heads are seeing are quite higher.. Once it boils cooling abilities fall off sharply. Vapor doesn't conduct heat as well as fluid . Since it is a fresh rebuild I'd bet that one or both head gaskets were on wrong.Just drill the old thermostat as George suggested or buy a new upscale thermostat that has air bleeds.Buy or barrow an infrared thermometer. You just point it at the surface being checked, pull yhe trigger, and it gives the tempreture of that surface. from as farr as 8ft. you don't get burned or dirty.I'd bet there is a big difference between the front of head and the rear.The coolant tempreature should gradually increase as it flows in the block rearward past each cylinder and then back forwards past each cumbustion chamberin the heads . The termometers run from $50 to $200. Craftsman has a dandy in their new 2004 holiday gift catalog for $149.99 . Laser indicater , pistol grip , back lit LCD and everything, it measures temps from -58° up to 1500° and they have another thermometer that reads temps from -58° to 1000° for $99.99 . You can use it to find dead cylinders by the exhaust temps.Bad spots in the radiater.bad bearings, low air pressure or out of alignment tires, hung up brakes,Anything where temp differences would be an indicater.
Almost every thermostat manufacture makes several grades of thermostats cheap to grand The better ones have the air bleeds allready incorperated into them ,are made from better materials , and are made so if they fail they fail in the open position. I know someone is buying the el cheapos but it isn't me.