I replaced mine a couple years ago. It was a pain but not super difficult. WCCC has some videos on how to do it. The biggest PITA was how brittle the box is, don’t force anything or you’ll end up breaking pieces off of it.
I getting ready to start this project on my 72'. After watching the video it almost looks like I can remove the heater box by getting access though the glove box opening. Can someone confirm this? I would like to avoid pulling the entire dash apart.
Today I removed the heater box by removing the glove box. I see that that WCCC has new heater cores but I also need a new box drain hose but they don't show that they have one. Any suggestions where I might find one?