Are you trying to use the main seat to run a backhoe or do you just want to be able to swivel around? How are you planning on dealing with the seat belts? They are attached below the seat base and I see a problem swiveling your butt and with no seat beat movement to match. Since the seat also pivots up, attached the belts to the bottom of the seat itself wil not work.
Boating catalogs/dealers sell swivel plates where two flat plates are about .75" apart with ball bearings in between. They rotate 360 degrees. If you rig something like this up, I recommend limiting the swivel to a 80 degree arc or smaller by installing some kind of mechanical stops.
