Fresh 318 , the good and the bad

Did you ever run a compression check on this motor? I’ve had a lot of the old 5.0 foxbody Mustangs and they are notorious for blowing the head gasket between adjacent cylinders. When this happens, it pressurizes the cooling system and pushes water back into the overflow tank and eventually causes the radiator to be low on water and overheat the motor. You can tell if this is an issue with your motor by running a compression check. Typically you’ll see too low cylinders adjacent to each other, usually the inside ones. Not saying this is your problem, but it has the same symptoms if you’re puking water out of your overflow tank when it’s sitting and warmed up.