the only thing I've seen that would work is a fifth wheel. That said, I have seen what you want done 2 ways:
The first belonged to a big logging outfit around here. It was a converted road tractor that had tool boxes, vise, welder, etc. up behind the cab, a short flat deck, then a 5th wheel mounted just behind the axle. Most logging equipment around here weighed about 30,000 or less back then, so he would have been pulling less than 50,000 most of the time. I never drove this truck, but I knew several guys who did, and they said it drove fine for what it did.
The other one belonged to a big farmer. It looked like a regular flatbed, but it had a square plate inset into the deck. When you pulled the plate up and off, there was a 5th wheel mounted to the truck frame under it. He had a lowboy with a higher than normal goose neck, then the king pin was mounted to a boxed section that came down off of that.
OOps, I said 2, now I've thought of a couple more. A welding shop had a short tractor, and they had a flatbed with a king pin on the bottom that stood on 4 removable legs. They backed under it and hooked up like a trailer, then pinned it to brackets on the frame to keep it from rocking and turning.
Also, there is a thread on here trucks that are convertible from tractors to dump trucks. I'm sure you could do similar, but that is probably more trouble than you want to go to.
On another note, I would lean heavily toward the 8100. I have owned a couple of those, and a single axle 8100 would not be working to pull 40,000; where a 4900 probably would.
Good luck, and keep us posted,
Mitch