Its true about having to check calibration against a mechanical gauge...Holley sender units and everyine made are read differently by Holley systems...
If you go to the sensor settings you can change the sensors offset to make the sensor accurate but you still have to do 4-6 month periodic sensor calibrations to ensure accuracy because 1 had a 125.00 sensor work great in the spring and crappy in the summer.....
From what I read if it s really running hot Id go over how you installed the head gaskets.....When I build my engines I take lots of pictures of the processes throughout this way if a problem arises I can go back to the pictures to verify I did the process right.........
Good Luck