BX tractors are great machines but probably not that great pulling a Box Blade.
Except in really soft dirt, you need 100 pounds minimum of weight per foot of Box Blade width to be effective.
For "nasty clay" 125 pounds per foot would be preferable.
I would do most excavating in slightly moist soil as I realize how tough clay is. I work in a job with access to steel easily, so adding weight would be pretty easy. I assume that is what you are suggesting? The piranha bar is nice, but I also want to use the box blade for my gravel drive. Not sure how they do in snow?
I want to use the blade to do some landscaping and leveling where we added trenches in the yard last year for some drain tile, and to level out some dirt bike trails.