Maybe the gauge in the car is accurate.The sender for the gauge is between #1 and 3 combustion chambers.The cts is in the intake manifold.It makes sense that the temp between the chambers is going to be hotter than it is up in the intake manifold.How many degrees I don`t know but maybe when the gauge says 220 the cts is seeing only 205.

 

Have you checked the wire coming off the unit for corrosion? Does the gauge move when you turn the key? If so you might have it shorting out at some point. My 91 350 had a improper ground, changed the wire, electrical taped it and replaced both sensors just to be sure.

 

This is very normal for these cars. The gauge in my 84 T/A reads 20* hotter than it actually is. "Assuming" my fan turns on at the prescribed 195* per the switch in the car. My gauge reads 220* when the fan kicks on.