In the context of the OEM cooling systems on these old vehicles Carl is correct. To the OP a 160 +/- is more than adequate for heating in the winter (ever stick you hand into 160 deg water?) and allows the engine to start dumping heat sooner. I run a 160 year around in my '38 and love it. The OEM thermostat on my '38 is a 160. The heater can run us out in the winter if I run it full bore.
Looks like a '59 came stock with a 180. I'd just leave the 180 in it full time since that's what it was designed for. If you're running the cooler 'stat in the summer due to cooling issues you likely have a partially plugged radiator of a lot of crud in the water jacket or both.
