My dad said his 75 had one but he scrapped it in early 2000s as the metal rotted so badly to my understanding the floor is obviously different by the pan is gone but the under cross supports should all be the same, with the wheel tubs being extra obviously as you can see, and the tail pan is slightly different? But you can buy a kit
No part of the wood floor is shared with the traditional metal floor other than the two small, short supports in the middle. The 6 cross supports that are used to bolt the bed floor down in the traditional 4 locations are entirely different. The supports are taller and mount wide side down. The perimeter panels are one off's and is the tail panel and front panel. The front panel and tail panel could be modified from a metal floor bed. The tail pan would take a lot of work to do though.
As I understand, only about 3000 of these beds were made in long bed, none in short bed. That's 3000 out of 18 million(?) Squarebodies made making them pretty scarce. And since these were a $100 option typically only used by small ranches/ farms that needed a wood floor for livestock, most have been used hard and are in pretty rough shape.
No parts are available except for the wood, bolts, and bed strips. When I last spoke to Bedwood, they stated they sell less than one set of wood a year.
It took me several years to find one, in any condition, besides people telling me regularly that their cousin, uncle, etc. has 2 in his field, etc.
Restoring the bedfloor and shortening it for a 6' has been the most challenging part of the build of my son's truck. I do have 2 extra bed supports, but that is the part that is least likely to be damaged or rusted.
The sheetmetal can be fabricated at roughly $200 a piece x 9 pieces, add a good 10 hours of skilled fabricator on the tailpan and front panel, add wood, hardware, etc and you are at an easy $3k on the floor alone if you wanted to build one.
I've only seen 3 on trucks in pictures, including the OP's and have never seen one actually for sale other than the one I bought which was in fairly good shape but still took $1500 in bodywork/ sheetmetal repair of pits, tears, minor rust, etc of the perimeter pieces.
Hopefully, you can find one in decent shape that can be repaired and as long as you have most of the pieces you'll be in good shape.
Good luck on your search, and if you hear of one anywhere and don't buy it, let me know and I'll go get it.