Good work man! I couldn't understand why the 3 part handles until I saw the side elevation view, showing the 3 sizes. What is your estimate of the base weight?
Dan - took me ages to balance the front tie/gussets [xsteel does the Centre Of Gravity calcs]; I'm workin' with whats available "free issue" at work... So the 50x50 SHS is OVERKILL and we could probably get away with 25x25 box, however I think that the weight bodes well for future additional componants similar to Ironminds Vulcan racks (dip bars etc)
I'm still waiting for my new home gym to be completed (rafters on - roof starting in the new year) so I have no where to fabricate/store; this will be more of a spring/summer project.