philo err Don,
I don't think the wagon would need a lot of reinforcement, use a single piece of 14ga steel with beads rolled in it, for bed floor and sides, leave the rear doors intact, just welded shut and window frame cut off, below the rear window, make a 1" thick box with 1" square tubing and 14ga, again with beads rolled in it, weld to rear of door post, make the back window panel from 16ga with whatever kind of rear window, a sliding rear window from '80's ford Ranger comes to mind.
Eddie