Did you test the Stant stats before installing them in a pan of water on the stove with a thermometer in it? I always do. You might want to test them now and see if they're working right before replacing them. Could be something else. If this is for an 8ba engine, Shewman's stats should be OK for cooling but not so good for warm up as explained in post #11. If for 59A or earlier, you might want to read these threads about Shewman's stats before buying them:
https://fordbarn.com/forum/showthread.php?t=112889 and
http://www.fordbarn.com/forum/showthread.php?t=143576
You can order Shewman's thermostats on the phone or by email at 1-610-933-6637 or
[email protected]m