Hot 454V8

[TJ2000]

Have a 71 SWB K10 stepside w/ a 454V8 and Manual 4 spd. trans., 4 core radiator, new fan clutch on fan, no thermostat at this time due to running hot. At highway speeds (60mph+) temp is fine, at slower speeds (offroad) engine tends to eventually overheat into the red zone. Also what is the purpose of the pre-formed 3/4" 90 degree bypass hose (I think people call it a bypass hose) that goes from the top of the water pump to the front of the intake manifold?? Thanks for any and all info...