Gus,
The frame underneath is I-beam, the uprights box tubing. The front one is adjustable so it is easier to mount the frame and crossbars in, plus, for an MC or 40, I would make a different front mount that is more of a saddle.
Part of the reason I did not go all saddle, is I hoped that by bolting the uprights rigidly to the crossbars, I would get a little more rigidity in case some idiot bumped it with the forklift.
![Laughing :lol:](./images/smilies/icon_lol.gif)
If you make it with saddles, make sure you can clamp the crossbar rigid, otherwise you could up end the unit if you hang both finals without an engine on the front, or vice-versa.
Not a lot I would change I guess so far, maybe brakes on a couple of the casters. I plan on putting some square tubing under each end to put the forks in when moving it around.
Lavoy