I had a over heating problem in my sbc before. Mine was due to a dead cylinder- I wiped the lobes off of the camshaft by not adjusting the valves right and the engine had to work harder, with more timing, to make power. I changed out the cam, added an electric water pump and it solved my problem. Just a shot in the dark, but it may run hotter than stock motor and bore due to the thinner cylinder walls from the .40 over bore.
JMO