Stone1
We have a lot of clay here to. The consistency goes from gooey to easy to work but heavy to dry and hard as a rock every spring. We are know officially at hard as a rock.
Don't have a box blade yet so I don't know if my tractor would pull a 5 foot box blade, you might have to take smaller "bites". Maybe this will help; I can pull a 14 inch Rankin ripper tooth sunk to the hilt as long as I go slow and have the tractor at about 2600 or 2800 rpm in low range on the hydrostatic drive. That rips through the clay and the lava, when it gets to a big chunk of granite or basalt we do come to a halt though. Love that tool, I hardly know where the pick axe I used to dig trenches with is anymore.