[Hevy]

Replaced mine yesterday. It's not that hard but take your time.
I pulled my bezel, dash pad, glove box, all air ducts, and then the box itself. Pulling everything out gives you more room to work and I wanted to clean everything before re-installing.
Before anything else remove heater hoses under hood.
There was one bolt inside the cab on the right side of the box and the three nuts on the outside studs. If you have the extensions and swivel you can remove all the nuts as is without removing fenders, etc...
Even after everything was removed the box was stuck tight to the firewall after 36 years so you might have to pry it off, don't break anything. One vacuum hose on the left and one in the middle (mine cracked and broke off), remove the temp. cable and remove the case from truck. While the box is out you may want to check the vacuum diaphrams to make sure they both work before you install everything again. To split the box (on my '77 with A/C) there were eight screws, three by the temp. door arm and five holding the halves together.
Split the case and there is the heater core. Four screws and metal brackets hold it in the case, remove those and swap it out.
Reverse everything and add coolant.
I'm sure I forgot something but that's it in a nutshell.