Nice calcs. I help to relieve the tension on the end of the spike by doing what I said. I welded a plate at the base of the spike to the frame, that the pole just slides under....sliding fit. That way the bending moment of the pole comes into play in distributing the load. Plus with a simple bolt dropped through the two, it is locked in place and installation/removal is fast.
I found that a 2 7/8 drill pipe works best having enough metal thickness (schedule number) and diameter. plus it's not too heavy to lift.