Older NORS Thermostats

Fitment listed not quite right. Factory manifold on 78 and older V8 had a bigger (outer diameter) thermostat. 79 and newer V8 had the same thermostat as any /6.
The center section in pic looks like smaller diameter than the similar design robertshaw
That one claims to fit the /6 but they don't. The center section is too big, hits the water jacket side of cyl number 1. This one looks like it's small enough it might actually fit.