The "official" 261 water pump has dual tapped holes for heater hoses, one goes to the actual heater and one goes to the bypass port on the thermostat housing. Other than that the 235 water pump from the same year will fit and work, you just won't have a place to put that hose. There was a water pump change in '55 from the 216 style with the plate to the late style with the impeller in the block.
