Welcome to CHR Chubbies.
There's not much chance of finding a 67 to 69 Z28 sitting in a field. There might have been one once upon a time but there's a few thousand guys who would have beat you to it especially in California.
You might be able to trade your El Camino for a rough one if the El Camino is in good shape. If you check on ebay you might find out what's available and how much they cost.