[mongocanfly]

If its showing overheat,then it may be..you really need a gauge on it..
But, did you fill the engine with coolant thru the top hose and let the air work it's way out?..
Sounds like you may have the engine airlocked..
Take the top hose of the radiator and fill the engine til you quit getting air bubbles..
Also do your heater hoses have a block valve or bypass valve? ..not all the time , but I've seen it happen, if the heater hoses are blocked off ,it wont circulate the water to open the thermostat
Your wiring harness/ecm should be reading temps from the drivers side head..that turns on the fans
You can use the pass side head for your in dash gauge..I drilled and tapped my pass head for the mech gauge sender for my dash gauge