My ect/eot's are normally in the 4 to 8 degree spread.

 

Running 90-95 MPH this morning, ECT was 186*-192*, EOT was 206*-211*. That was for two hours in 28* air temperature.

 

I recently replaced my T-stat. My ECT climbs much quicker and stays above 190F now. I used to run 184-186 and climb very slowly to get there. The motorcraft T-stat with o-ring is only $20 from Tousley. I think you should change the t-stat with every coolant change. Why not, it is so cheap. Mine was a bit dirty.