I agree with Al but will add a couple other things.
You need to have a thermostat in place because it has a valve that blocks
a bypass in the head which, when open, recycles coolant within the engine
rather than routing it through the radiator. If this bypass is open, you will
not get full flow (only partial) through the radiator. If you insist on running
without a thermostat you need to plug that bypass, which you can probably do
with a 1/2" rubber stopper from chem lab.

Also if the total cooling system is exonerated, you should check to make
sure that the exhaust flows freely and that the timing is not retarded.
Both of these can make an engine run hot.
--
George Downs Bartlesville, Heart of the USA!