Remove the hoses (catch the antifreze). Unbolt the outer housing from the inner (the outer is in the engine compartment the innner is inside the car, the studs are welded to the inside housing and the bolts are on the engine side). Remove both inner and outer housings, pull the heater core out from the inside of the car, get a new core (it is not easy to repair the old ones and new ones are around $45), and put it back together (do not over tighten the hose clamps!!)
Replacing the heater core or fan on 67-9 GM F-body A/C cars is a real pain. The two most difficult parts to get to are the lower-right mounting bolt and the lower heater hose. If you have small hands and arms you may be able to reach the lower RH attachment by loosening the lower fender/wheelwell and reaching up inside. The lower heater hose is a bear. Before I bought mine someone had drilled a 2" hole just in front of the nipple to ease access. I would not do this unless the wheelwell was removed from the car. You do not need to remove the evaporator case on the engine side of the A/C system, but you need to remove the two nuts on the engine side of the case. The top one is easy, the lower one is more difficult since it is buried behind the engine. Getting to all of the cables and ducting attachment screws can be challenging if you do not remove the ash tray, radio, and glove box. Be careful when taking off the cables since the attachment points are fragile. It was also worthwhile to spend an extra hour and re-seal all of the ducting and deflector doors in the case with foam tape. It is amazing how much better it works with all of the leaks fixed. Good luck.
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