Here is the finished product (untested as of yet).
I added some stainless shims to the 2.5" stinger before hammering the 2.5"-3" adapter on the 2.5" stinger. I then welded the 2.5"-3" adapter to the end of the Curt 2.5" stinger only. It would be easier to just weld the adapter to the stinger at the back and the front of the stinger making it one piece but that weld around the stinger would be a perfect spot to crack and snap the shaft.
I also made a shim top/side for the 2.5"-3" adpater in the trucks reciever hitch. The approx 16ga shim made the fit perfect with no up/down or side/side slop. The 3" reciever fits pretty loose to the adapter and my 3" weigh safe 180 hitch for my taste.
Stainless shims added to stinger.
Stinger/adapter effective length.
Welds on the pin sides only. Not really worried about the adpater slipping on the stinger I just wanted something to retain the adapter.
Finished product.