I ask out of curiosity why you're replacing it? I know you say it didn't work "good enough" but i'm just thinking out loud that there may be another issue at hand. That looks like a late model engine by the casting marks on the heads. If you installed an early model water pump on it, it's the wrong rotation and will not cool the engine down. I believe around 1988 or so when the serpentine belts were introduced en masse that the water pump impeller was switched to spin to match the proper direction of the belt. So you're probably going to have to install a reverse rotation water pump to match your "early" belt and pulley set. Also, are both of your fans acting as "pullers"? I installed a set of similar fans on my friends '84 K20 and the company had the wires switched in the harness to one fan connector so one was pulling and one was pushing. We figured it out in short order and saw a marked drop in engine temperature. The only other thing I offer is if the engine by chance was ever apart that it's possible the wrong head gaskets were installed on the engine. The passages and coolant routing are different between early and late engine designs and although the gaskets will install on either combination, if you put early gaskets on a late engine, if will "cool" but once you get motoring it can't keep up because the coolant flow is essentially dead heading in the block and the issue presents itself. Worst case scenario that last one is. but I've had many big blocks in different combos and when it's all correct they usually run quite cool.
