how to remove and install the heater core

Well the first thing you'll need is a good friend, a filled cooler, some time, and slow fuse. Replacing a heating core on a 1978 Monte Carlo is a job only a contortionist would love. The heating core is a small radiator that's usually buried under the dash near the firewall. The part is relatively inexpensive, and relatively easy to replace once you get there, but as Butch said to Sundance it's the fall that's gonna kill ya. Getting through the dash is a tedious process that is too complex to adequately explain here. Get yourself a Chiltons (and with a Monte Carlo you may be lucky enough to fine one at your local library),a good friend, and block out a solid stretch of time when no puntable pets will be wandering by.