I couldn't see why you couldn't rig it like that. My HJ 308 (original manifold) has an outlet on the thermostat housing that only attaches back onto the manifold under the choke. My heater hoses (both) run from the water pump housing to the (A/C) heater box. I would suggest that the hotest water for the heater would be from the thermostat housing as the water exits the engine in that direction towards the radiator.
Regards,
Dave
Bookmarks