Are you talking about adding factory air conditioning setup?
If so, I have done this, where I replaced the non-Air dash with an Air dash (involves unbolting, cutting spotwelds, and rebolting, rewelding new dash in) and also the holes in the firewall are different. I cut out a section of my firewall and welded in a section of firewall from an air cab.
If you are talking about aftermarket Air conditioning, I would think that there would be miminal modifications needed. Someone that has put in an aftermarket kit could answer that better then me!!
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John Fabris
john.fabris@autodesk.com
1967 C10 SWB FleetSide Web Page
1971 K5 Blazer 2WD (basket case, but give me two year or so.)