See #18 above, spot on.
I agree with All_talk assessment, I was worried about the weight too but I talked to my fabricator before about the leveling system in gonna use so he suggested this, can you show your fabricator my levellers and ask his opinions?The threaded blocks like the OPs stand are a very good method for a steel stand. They allow for proper thread engagement and spread the load to the walls of the tube. A welded in nut could be OK depending on how its done. Two things I would avoid are, any configuration where the weld itself is carrying the weight. Or the nut in the middle of a horizontal face of a thin wall tube.