Stock heater vs After market

Had a 75 Duster (318, 3 speed, no ac) back in my college days (late 80's)and after having the tank heater replaced after about 15 minutes the heater would have the car nice and warm. So my vote would be get a used heater box. I would think you could pick one up pretty reasonable here on FABO, put a DMT reseal kit in it and a new heater core.