So you only swapped the heads? WHY? Without the cam and related hardware you haven't gained any HP and probably lost some. Thermostat won't make a bit of difference, its only function is to open when it hits it's rated temp, it DOES NOT control overall engine temperature, that is a function of the size of the system. The aluminum heads allow you to run lower grade fuel since they lower combustion chamber temps by passing the heat through to the coolant faster than the iron heads. Because of that any unburned fuel will stay in the cylinder longer and cause carbon build up. The only way to stop it is to get the rest of the parts for the Vette engine and put them in, then swap the chip for a Vette chip and any other hardware that is different. The head is designed for those parts.