OK. Having bosses on both sides of the housing maintains the source for the stock heater hose loop. But running a line from the driver side of the thermostat housing (through the intake) to the block drain hole doesn't provide for any flow. When the thermostat is closed the water pump impeller is pressurizing the coolant in the block - pushing against the thermostat awaiting for it to open. A line running from the thermostat housing back to the block would be under equal pressure when the thermostat is closed - and subject to minimal if any flow when the thermostat opens.
The stock heater loop is attached to the inlet of the water pump for this reason.
