Part of my engine had been rebuilt, but they forgot to install a new thermostat

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About 2 years ago at Al's automotive, when they changed out my SOHC to a DOHC, because I had an oil gasket leak. It changed the location of the thermostat from the front area of the motor. Where the lower radiator hose goes. Now, to behind the distributor. I couldn't find a mechanic who wanted to do it for me. Probably why the exhaust manifold cracked in the first, place. And, my car began to guzzle gas up, then be fuel economy. OR IS IT JUST PLAIN TOO LATE, FOR MY LITTLE BABY!!?