Tom:
I measured my cradle yesterday.
It has a base made out of 3.5 Inch U channel welded together into a rectangular frame. The outer dimensions of this base are 10 feet by 5 feet. On each corner of the base welded vertically up are four steel tubes 30 inches high. the tubes have an outer diameter of 2.5 inches. Supporting each tube are two pieces of angle iron welded to the tube 2 feet up from the base and then the angle is welded to the base 2 feet away from the tube. There are two supports per upright tube. One runs in the 10 foot direction of the base and the other in the 5 foot direction. The base then has two cross beams running in the 5 foot direction support for the keel. They are welded u channel right each in about a third the way from the end of the 10 foot direction with a 2" by 8" board attached over the top of the 2 cross beams and the end of the base. The board runs in the 10 foot direction.. The pads are the threaded supports from the top of jack stands and fit right down the tops of the tubes. IN this way they can adjust the height of each pad.
I hope this helps.
Mark