440 getting hot

So you have a leak from combustion to coolant. First place I would start is a compression and leak down check to determine the cylinder or cylinders that are leaking. Once you know this you can determine the areas that need inspected. I don't know the history of the engine so unless there is something cracked the point of concern is the head to block area. When you pulled the heads were there indications of where the coolant was leaking? There are limits in the manual on deviations of the mating surfaces. Did you straight edge the block and heads to make sure they are true, or were they checked at the machine shop? Did you follow the torque procedure during reassembly? Were the bolt holes in the block cleaned out, dirt fluids etc can cause incorrect torque values. Were the surfaces clean and dry? What type of head gaskets were used, composite or steel shim? Aluminum or iron heads? Just trying to get some more information out to help you. I would not run it anymore until you correct this issue or you will create other problems. The combustion to coolant leak will cause temp issues, and needs fixed before worrying about water temp at this point.