First of all welcome to the bolt.
To the best of my knowledge, 59 Chevrolet trucks came with a wood floor in both the stepside and fleet side models. It was not uncommon for a purchaser to put a steel plate in the bottom of the bed to protect it or cover the holes after the wood rotted out or got broken out. I have a 73 K10 that I did that to while the truck was fairly new to protect the wood. I would guess that you would find the wood bottom or what is left of it if you looked under the truck.
Someone might know something I don't though.
Jim
