Fortunately the largest hole saw I had was the right size to cut the center out of the one flange. None of my drill presses has the power to do this job, this is where my mill/drill comes in handy.
The two flanges mated together for rotation of the hoist.
I took the center cut out and turned a socket to fit into the end of the 2 1/2" pipe. This should result in a very solid connection when welded.
All the parts required for rotation and solid socket installation of the hoist.
So hopefully I can prepare the pipe, and strengthening ribs, for the hoist base tomorrow, and get it assembled.