What thermostat are you using,the one that screws into the engine block,manifold or the other that's go into radiator?Thanks

Intake manifold where the upper hose connects to the manifold

The intake position is the most consistent, however if you have non header type of exhausts you can use the head position with little difference.

I have always preached that the intake is the only place but head locations usually if not always are heat tainted by header radiation. Iron manifolds usually do not exhibit this to any great extent.