84,
You could have something outside the cooling system causing your problem, especially since you say the engine is rebuilt. What springs to mind is a blown or leaking head gasket, leaking combustion gases into the coolant. This will make it heat up real quick.
If it is only the cooling system itself, the fact that you ran cooler without a t'stat indicates the t'stat was causing a resriction to water flow, so the pump may be inefficient or the wrong one. More flow=better cooling, & this has been proved time & again by companies specialising in cooling.
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