There may be a thermostat but you should NOT run without one because
that short circuits part of the cooling system. The valve flap on the
bottom of the thermostat closes off the bypass in the head when the
thermostat is open.

My guess is that your engine is not overheating, but your temp guage is
reading wrong because of the voltage regulator on the back of it being
screwed up. If your gas guage is reading fuller than it should be or you
run out of gas when it shows 1/4 tank or so, that pretty much proves it.
I had this problem with my 73 145. The regulator is a small plug-in item
that is pretty easily replaced once you get to it on the back of the dash
panel.

If you can stick a thermometer into your radiator and check the REAL temp
when the gauge says it is overheating....
--
George Downs Bartlesville, Oklahoma