The heater core is replaced by removing the inside heater box (inside the car); all you need to do in the engine compartment is to remove the nuts from the inside heater box studs that protrude through the blower housing flange. See the '67 Chassis Service Manual, page 1A-4 and 1A-5. The fender or skirt removal is only necessary if you need to remove the blower motor (page 1A-3).