Dear Tom;
Here (Apple Valley) the temp varies from +103 in the Summer to as low as +6 in the inter. However; these are the extremes.
You only need plug the holes in the thermostat housing/water pump, unless you run a heater. It shouldn't hurt to leave the ones in the manifold open.
If you run a 180 degree thermostat you'll be just fine even in winter.
Another note: Make sure the fuel filter is on the right side away from the exhaust too. Easy way to do this is to get a fuel pump with one built in. GM made one for the early years stock.
Good luck, John M...
