welcome to the board.
The VIN decoders you've looked at are wrong. You can't tell a chevy's bed style from the VIN. However, if you have the SPID in the glove box still, you CAN tell what bed it had by looking at the model number.
Here's the only 100% acurate VIN and model decoder (for chevy) that I have found on the web.
http://67-72chevytrucks.com/Chevyear.html