I had a '71 K10 Suburban that had a factory plywood rear floor,the marine quality stuff soaked with resin,and 1" thick..
It was a bitch to cut it up and get it off..
It was about the only thing holding the truck together !..bought it for parts,when I cut off the roof,I kicked the rear quarters and they fell right off..drivers side door had the hinges rot out of the cowl,it was laying in the back of the truck when I got it!..
I've seen several 1955-59 Suburbans with factory wood plank floors in the past too..