thermostat compatibility

[Damien B.]

Hi I have a somewhat dumb question.
I was looking around and saw that there is a distinction between thermostat for engine from before and after 1979.
What are the differences between those and are they completely incompatible ?

 

[Darter6]

In the USA Chrysler discontinued the big block engines for the 1979 model year.
They also re-designed the small block intake manifold using a smaller diameter thermostat.
Thus the difference between pre and post 1979.