overhead a/c removal

[BAD1916]

I have a 71 gmc with roof air but the plenum is sagging and broken. Recently a lady ran into my dad's 70 Chevy suburban while parked and the insurance company totaled it. My dad bought it back for the engine and Trans and is letting me pick it apart for mine. His is also an overhead a/c and in much better shape. Is there any trick to removing these without causing damage. I looked real quick but haven't started taking it apart. Thanks in advance.