I recently had a major failure on my 3.5 V6 to the extend that I had to replace the engine with an import. I fitted the new engine after replacing the cambelt, thermostat and radiator cap. It started 1st time, and is running incredibly smooth, not even a lifter noise.
When I took it for a drive this weekend I noticed that it got a bit hot (3/4 of gauge)

when under load, and I sure do not want to replace another engine soon. Can anyone tell me if this could be due to the thermostat's jiggle valve not being in the correct (top) position?
The radiator, hoses and fan seems fine and the water level in the radiator stays constant. I also verified that the thermostat opens.