Need a thermostat for my rebuilt EA71 in a ‘78 Brat. Both 195 and 180 degrees are listed as the OE temps. What Is the correct operating temp of a ‘78 Brat?

You could use either , best suited to what you do and where you’re located. But the 180 is fine enough.

 

Some folks get a little antsy when their old 70’s Subaru sits in traffic and the temp goes up kind of fast. But that’s usually the fan. 

i can drive in rush hour traffic at a maximum of 105F with a 160F thermostat, if it's any hotter outside, i start overheating (with electric fans going)

but it sucks in dead winter when it gets to 25-30 degrees, the brat coughs for about 10-15min, by the time it fully warms up, i'm already where i need to be lol.

also keep in mind, i removed the mechanical useless fan and installed 2 electric fans on a switch to survive the summer heat here.