OK.. got the aluminum 2" tubing and 3-1/2" aluminum pipe in. The pipe then gives you a 4" OD. Had to to use a 4" hole saw to get the inside of the PVC pipe round and slightly oversize so it would slide on the pipe. In the end it was still a tight fit but it went together. Will be milling the end plugs for the all thread that holds it together tomorrow. Pic attached.
And I got the bed length wrong in the above post. It's 81 inches with 6 frames