I downloaded a story about building one of these 'New One Tonners' many moons ago. I would not be surprised if they were capable of towing 3.5tonne, but braking and stopping with that load could be interesting.
Back then, the rules and regs regarding towing and load specifications were not as stringent as they are now.
I can't find the source of the article now, but photos of the chassis look more substantial than the Crewman or Falcon ute, be interesting to compare with a Holden production single cab tonner..........
"Inside the trimmed cabin, with the front seats flipped forward, evidence of the new chassis can be faintly seen as square-section carpeted bolsters in the rear floor. Each 'prong' of the new chassis is spliced through the sheetmetal of the original floor pan and extends to the front floor of the cabin where it is incorporated in the Holden front chassis rails. Bracing this far forward spreads the loads over a larger area, resulting in a very stiff vehicle. Also visible is the storage cubbyhole, and the rear speakers and lights."