AFAIK, NOS is all that's available, but there always seems to be a few out there. For now.

If the car is to be a daily driver in MI during winter, I guess maybe you need one that works. But if you are only going to drive the car in above freezing temperatures, you can get by without one, by using only the exterior parts if you want it to look original. The spring and thermostat have a relatively short life span in my experience.

If you find a kit, be certain it is for 340 and study the drawing in the factory service manual to be sure it has all the needed parts...