Any chance you have a GM thermostat in the Ford housing?

Chevy's fit the Fords, but the problem is, when the Chevy stat is open it does not have the button on the bottom to seal off the bypass hole in the bottom of the thermostat well like Fords do.

When that happens almost half of the coolant gets bypassed and doesn't get cooled by the radiator.

Two fixes.
Run a Ford thermostat.
If you can't find the temperature rating you want, you can still run the GM stat.
Tap the hole in the bottom of the thermostat well - 1/4-NPT if I remember right - and screw in a flush allen headed pipe plug.
Use Neverseize so you can backtrack if you want.

I did this little trick in a Cleveland 400-M.
I know the Windsor stat housing is different, but I'll bet it has the same bypass system in it.