StuartBJ did you get this sorted. I see no problem in connecting that hose right to the header tank. That's the way mine is except I also have a heater core in there as well. My header tank also has a hose running to the bottom of the radiator as well as a purge vent line running to the top of the thermostat housing (highest point) and a cap vent line running to the overflow tank.
In the attached picture of my tank that hose you speak of goes to the side brass fitting on my tank. The radiator goes to the bottom larger fitting. Thermostat hosing goes to the top brass fitting. Overflow goes to the top aluminum fitting on the cap.
M120 into 560SL (With Pictures!)