Success!
I had tightened up about the front third before I checked for square, so I loosened up all of those a bit. Then I took a measurement and I strapped a ratchet tie-down on the longest diagonal. Still out nearly an inch.
Ratcheted it up a bit, remeasured and now I had gone a little too far, I let back two ratchet points, gave the other diagonal a light tap, and now it was near perfect. I started to tighten up the bolts. slightly sequentially, double checked for square, completed my tightening.
In the end my diagonals on a 5x8 foot kit measured 113 3/8" by 113 7/16".
Don't think I can get it better than 1/16 of an inch difference.
Now the bed is ready to be located, (I already spiced a piece on it) bolt holes marked, yada, yada, yada.... Here's to hoping tomorrow afternoon is again warm enough to get some more done.
As always your friend in Tears!
Eric