the thermostat is or should be under the piece that the upper rad hose is attached to. This piece is bolted to the engine block usually with two bolts, has a gasket under it, and then the thermostat.....take it out, it should be closed, put it in a pot of cold water and bring the water to a boil...it should open, if it does then the problem is either the rad itself or timing, but if the engine was not knocking using todays gas when under load, then I do not think that is the problem.
