All I can tell you is that I pulled a unit with a heater core and A/C core in it for a guy on the west coast about a year ago. It was a bitch of a job and a good thing it was in a junker. Good thing that truck didn't need to run again :lol: . that being said, it must be dooable since that guy did it. He said it was going to be a bitch getting it out and it was. However, if the shop mechanic can do it, you can too. It will be easier if you can find a shop manual that goes thru the proceedure. I just looked it up in my 83 shop manual and it would have been easier if I had done that for the 78 one I pulled.
Basically you have to disassemble the dash and remove the heater core cover , duct work, take apart the plenum that the A/C evaporator. There are bolt on the inside and outsice that need to be removed. So like I said, it's a bitch and you have to disassemble a lot of the dash.
Good luck,

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