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I'm assuming that the engine cooling fan is coming on but the engine is still overheating. If the fan is not coming on, you need to confirm that it is functioning before proceeding.
I'm guessing that it's ok at idle or light load, but its overheating under load or on the highway.
There is a known issue with the headbolt threads on this engine. Although you have torqued them, the threads start to pull out and the clamping force of the headbolts on the head decreases and you get combustion leaks to the cooling system. The only fix for this is to pull the engine apart and repair the threads (GM dealers were issued a kit for this ) or to replace the block.
You might be able to detect this leakage by removing spark plugs and putting shop air in the cylinders while watching the cooling system for bubbles. There are also combustion gas tests that can be done on this engine to confirm this leakage.
Sorry to be the bearer of bad news.
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