Temp. Reading on my poly. Eng.

The problem I am having is the temp at the thermostat is 180 w/a 180 thermostat.
The temp. at the temp. gauge sender is 150 checked w/a inferred heat gun. The 2 spots are 30" apart, how can this happen on a eng. that has been running for 1/2 hour or more.
I noticed the coolant enters the block at the front of the eng. then travels up into the back of the heads then travels back to the water pump threw the heads and back to radiator. As you can see the sender is located near the back of the intake manifold.

I would like to install the temp. gauge sender near the thermostat housing so I get a more accurate reading.
What have you done on your Hemi/Poly?

 

See if you can find one that'll fit, or something that'll bridge the sizes.