Are you talking about the big factory heater/AC combo unit? They're not
that rare, really. I've got three of them in the shop right now.
Normally the grilles across the front have broken fins, so if you find
one with the front of the case in good shape they're probably worth
grabbing, since they aren't being reproduced. Someone who's doing a
restoration would love to get a good one, but generally speaking, for
someone just building a nice street truck, most won't want anything that
large anymore. You can buy custom kits for this generation of truck with
components that are much smaller and more efficient anyway.

Keith