Hot but Stable - Clutching at Straws

Check the gauge before you go any further. Get one of those infrared thermometers and check out the actual temperatures, if everything you've put on the car is working well it shouldn't be running hot. I like the infrared thermometers vs another mechanical gauge just because you can check the temperatures at multiple places to identify problems, like checking the radiator vs the hoses to check the thermostat, or even "watching" the radiator to see when the thermostat actually opens. You can even check the water passages at the front and back of the block (by seeing how hot the surface of the engine is where the ports are) to see if the water passages are blocked. The intake manifold where the water ports seal is a good place to check, especially on an aluminum intake because it rapidly becomes the same temperature as the water.

Could be the water pump, "generic aftermarket" parts aren't know for their quality in most situations, could just be that its a dog.

Who put the engine together? It could also be that one of the water ports got plugged, or that a gasket was installed incorrectly and blocked a port. This is one of those "check everything else first" kind of problems, but it does happen.

Keep in mind that even though the system kept the engine cool before the rebuild more power means more heat, so it may be that its time for a larger radiator as well. I wouldn't think so, there's plenty of cars out there running 22" radiators, but its something to consider if you absolutely can't find anything wrong with the system.