My 78 lariat started leaking from the air box drain tube, I knew it had to be the heater core. Let me tell you, that was a chore to get apart. The whole heater box must come out from under the hood and the a/c evaporator has to be removed from the box to get to the heater core. I noticed my evaporator was turning green and rotting at the bottom too, so I replaced it too. The heater core was only 30. bucks, but the evaporator was $300.00 !!. Just got her back together today, no leaks and ice cold a/c. (I had some R12 laying around from the good ol days) Boy am I glad to be done with that!
Non a/c trucks are easy, beware if your truck with a/c starts leaking from the core.