The M103 has bleed ports on the head. There are a couple of bolts on the surface of the head beside the valve cover rim above where the stub hose from the water pump meets the head. They take a 14mm socket. Some engines have a thermo-vacuum switch in one of the ports.

Consider filling with coolant before fitting the thermostat to minimize the amount of air trapped to that height.

That lower hose looks like it'll hold a quart of air.

With the right hex key you don't have to remove the distributor cap to remove the heater line clamp.

Sixto
95 S420
87 300SDL