This is a good question as i'm wondering myself; My engine generally does not get hotter than 180. It stays around 170 most of the time regardless of load or outside temp.
i'm pretty sure i put in a 195 thermostat which is what was called for in the service manual.
As far as temperature, i would like to experiment at 200 or even 210 because i feel that more temperature equals more mpg. But i guess the real again is what is the the best temperature for a SBC. Or what temp was the SBC designed to run?