Does it use a standard auto style fill port, or something more special? If its a standard, you could probably look up a cheap locking gas cap for an 82 pickup that would fit. Chevy had two styles of fill port, one was a key turn round one that had no fill door, the other had a recessed filler door and used just a simple thread in cap.
Here's a list of gas caps for comparable 82 Chevy trucks. I think one of these little buggers would probably fit. The WalMart Super Centers have the FRAM brand ones, or comparable in some of their automotive departments.
Gas Caps for 82 Chevy Pickup