180 or 190 thermostat?

The max hot idling temp will depend on the rad, fan, shrouds, etc. Changing the stat will not change that, but it should buy you more time from when you first come to a stop 'til the temp gets high. More or less power depends somewhat where the air for your engine is coming from. If you have a cool air kit where the outside air dominates the air entering the carb, then hotter may help power. But if all the air entering the engine is coming through the radiator, and the air temp is heated by the rad, then lowering the stat temp at cruise MAY cool the air down a bit (depends a LOT on the radiator design), make it a bit denser, and perhaps give better power. There is no sure answer....you just gotta look at your set up and try it.