I wonder what the difference in tractor weight is between his Kubota and the old Ford. Sounds like he's stepped down in tractor weight.
I pull a 5' BB with my JD2210. I'd guess the tractor weight would be around 1200lbs. I have filled rears and R4 tires. If you get too much material in the box it'll spin and a slight lift of the box stops that.
There's no magic in this, it's simple physics. If the drag resistance of the box exceeds the weighted traction the tractor can apply you spin out. At that point you have to either increase the weighted traction or decrease the drag resistance, hence slightly lifting the box, which accomplishes both.