Who is doing the diagnosis for this problem? How about doing an over night pressure check of your cooling system? The coolant has to be leaking from somewhere. If you did a block, the radiator should have been replaced, this is standard procedure. there has to be a hose or junction. maybe the heater core job was poor and this wiped out the motor. I have seen this before. Heater cores are easy to crack when installing them (I have accidentally done it myself ) Do your windows ever fog up, especially with the defroster on? ( heater core leaking symptom ) Good Luck!