Respectfully I know some people believe the substitutions are OK. I've been dealing with the substitutions for 40 years, I wish the solution were as simple as that. Punch the numbers of any substitute you are considering into eBay, see if there's a picture you can look at, and see if your substitute looks like the picture below. The manufacturers have substituted and cross-referenced thermostats that don't work. They've been doing it for 40 years.
Many Ford vehicles have been equipped with the "high flow" style thermostat over the years, I've included pictures of 3 of them. That part of the 351C thermostat is not unique. But only the 351C thermostat has that copper cup attached to the bottom having the proper dimensions (height, shape, diameter and distance from mounting flange) to fit snugly inside the hole in the brass plate when the thermostat extends downward, plugging the recirc hole at the right time, and allowing the thermostat to move up and down, adjusting temperature, while keeping the recirc hole plugged.
In 40 years I've never seen a substitution that comes close to performing the job properly.
A member of the 351C forum wrote he was able to find 192 degree thermostats, but not 180 degree thermostats. 192 degrees is actually the oem thermostat rating of the 351C, so its OK. But you should change your thermostatic fan switches to complimemt the thermostat if you go that route. I can help if anyone needs advice with the thermostatic fan switches.
I'm sure the shortage of thermostats is only temporary, Flow Kooler will make some more. I have seen a few people admit on the internet that they have hoarded a large stock of thermostats, so that explains the problem.
