depends is this a A/C car?
changing a heater core on a non-A/C car is easy.
changing a heater core on a A/C car is an exercise in aggravation.
for an a/c car you have to take the box out from behind the dash. If you took the dash out completely it would be easy. but nobody wants to take the dash out since that is more work, A lot more work.
so you have to work within the confines behind the dash with a thin, weak, 40+ year old fiberglass box.
you have to disconnect the A/C system, all the vaccum lines, pull the glove box insert out, pull the dash board re-enforcement bar out from infront of the box, to gain access, there is electrical connections to the Ice up sensor, a mechanical connections to the climate controls for the hot/cold flapper. you have to go into the engine bay there are either 2 or 4 nuts that have to be removed(i can't remember off hand) the Drain hose, from the bottom of the box that must be CAREFULLY removed or it will snap off.
then there is a screw holding the intake hat to the cowl.
i would recommend taking off the glove box door at this point
now you have to figure out the sequence of movements that like a combination lock will allow the heater core/ac box to drop to the floor board.
once out, you pop open the bottom of the box and as luck would have it the heater core is the first thing you will see, it will require some more disassembly to get to it but nothing big.
the new heater core will need all the foam from the old heater core removed and reglued onto the new one.
now is your chance to clean out the box as well, i'm sure 40 years of squirrel acorns will no doubt come out.
with the box all back together if it wasn't damaged when it came out you go through the combination lock movement sequence pushing it back into place trying very hard not to snap off the stud mounts that will go through the firewall to secure the box again. then put all the electrical and vacuum back on and tighten up all the screws and nuts.
it took me a full day to get the box in and out without damaging it.
another day to completely service the box inside and out and fix any cracks and damage with a fiberglass repair kit.
these boxes were constantly exposed to moisture from the evaporator and heater cores as well as outside air, the interior flappers are basically unpainted steel that had a light cadmium coating on them so its not uncommon for these to be rusted shut or rusted away especially the small flapper at the bottom of the box.
you have to work very slow and its extremely labor intensive there is no room for a second person when installing or removing the box.
I highly recommend removing the passenger seat as well.
