There are two crucial tips to remember when replacing a GM heater core. The first is to make sure the A/C system is drained completely. The second is to remove anything that blocks access to the heater core, which may include the battery, the inner fender panel, some ductwork, the glove box and the passenger kick panel, before attempting to remove the heater core itself.
Draining the A/C system is especially important for GM cars built on or before 1969, because the A/C pipes pass through the passenger-side inner fender panel and directly block access to the heater core. This isn't a problem for models built from 1970 onward, as reported by Super Chevy.
The process of replacing a GM heater core varies depending on the model of GM vehicle. Although GM manufactured many models throughout the 1960s and 1970s, it used only a few heater core variants during this time period, so it's possible for two completely different GM models to have the same type of heater core in place, as stated by Hemmings.
In an emergency where no tools are available, car owners can apply two temporary fixes to solve a heater core problem. One is to pour a sealant into the radiator and let it circulate until it plugs the leak. The other is to physically locate the holes on the manifold and water pump and fill them with sealant.