Well, in the near term I am going to be using it to carry 3 foot tall by 4 foot long concrete whistles. I could pick them up with my front end loader, but I think this would make it easier to slide the whistles together. They weigh about 800 or 900 lbs apiece.
What is a concrete whistle?
Back in the day, instead of metal whistles, they would use clay covered in concrete for the whistles. They last a long time. Our best estimate was that these were installed around 1960 or so and they are still in great shape.
OK, culverts. Never heard them called whistles though. I have heard them referred to as "jars" or "tiles."
The forks look like they would be fine. Seem to be heavy duty. Go for it.