The box is made of fiberglass and can be repaired if you have all of the parts (nothing missing leaving a gaping complex shaped hole).
Other than that all full size cars share the same A/C box. Four doors, Convertibles, Wagons, and Two doors are the same from the fire wall forward. As such any full size Chevy can be a parts donor. Additionally the same parts were used by Pontiac, Oldsmobile, Buick, and Cadillac (B and D body Caddys). If you can find a junk yard with cars this old in them (I found a bunch while driving through the Appalachia mountain states; especially in Western North Carolina and Northern Georgia) then you can probably find the part you need.