Got the camera running & took some better pics of the stand.
The way this one's set up:
The ring gear has a shaft collar welded to it with 2 keyway slots in it.
The shaft has 2 matching keyway slots in it, 2 allen set screws secure them together.
The lock collars outside the box on the shaft are used for end play, kinda like thrust bearings.
The rusty looking piece between the shaftcollars is the outer support & lock. It's ID matches the shaft OD, it has a slot cut in it with a bolt, if you tighten the bolt it cinches the shaft to keep it from turning.
The bottom of it is hooked to the ram of the 5 ton jack. If you have a engine side mounted sometimes you need to jack it up so there's vertical clearence to rotate it.
THAT SHAFT IS ABOUT 3" OF SOLID STEEL! The thin part of the rotating end where the bolts go through is 1 1/4" thick.
My thoughts are FWIW, weld a shaft collar keyed to match your gearbox to the tube, fab up a mount to secure gear box to upright part of stand. Put a grease fitting in the outer tube and maybe a brake. The brake on mine seems like overkill. I've never used it, the gearbox on mine is a worm gear & holds things ok for me, 6BTA is only about 1000 Lbs though.
James