I believe that a wood bed floor IS pretty rare (Only one I can recall seeing was on a '73 crewcab farm truck) on a '73/'87 Chevy truck.
Although I don't consider myself to be an "expert" on Chevy trucks, I don't think that many of them were built with wood floors - especially by '78.
What sort of shape is yours in? Stepside or fleetside? Wood bed kits are available if you need to repair/restore yours - I picked up a catalogue from a parts dealer here in Canada and saw them listed there.
Dixie, you are wrong. It was an option, and my 77 K/20 fleetside had it. I wish it still did, but I too couldn't find a wood kit, but more so than the wood, it was the edges of the bed floor, made of steel, that I couldn't find. The "strips" that hold the wood in, are the same as step side beds. The reason (I was told) GM offered a wood bed floor on a fleetside, was to target farmers and ranchers. Livestock, has a hard time standing on a steel bed floor, but you could put livestock on a wood bed floor, and move it from pasture to pasture, or anywhere else for that matter. YOu can fit more livestock in a fleetside, than a step side.
The GM Factory RPO for the floor, is E81, Wood Floor Pickup Box.
I have a list of RPO codes on my website, if anyone needs some.