if the heater core is bad - bypassing it is easy..

there should be a connection coming out if the back of the cylinder head... and the one coming off where the lower radiator hose attaches to the engine... connect both of those together and the heater is bypassed..

I'm curious on how the models without heaters like the panelvans were done

apparently the UK spec 120Y panelvans had no carpet, no seatbelts, no stainless steel trim, no heater, no radio, no cigarette lighter..