Hose and pipe combo that runs along the manifold side feeds the heater core, usually starts at/near the thermostat housing. Then there's another hose from the firewall to cyl head that returns flow to the engine. Thats your bypass route.

And if it was me i wouldnt retorque the head for fear of breeching the seal of a head gasket thats already been crushed to specs and set. Whats the torque specs on the 300D cyl head anyhow.... somewhere around a whopping 76 lbs??