Kool!!! Central Valley Kalifornia!!! Love seeing more local folks chiming in !!!
No prob here with a BushHog SBX 48" box. As far as weight ... I have successfully hauled pattets with 700# of pipe (pipe is easy to calc the weight that's why I can quantify it).
Tractor geek note: I have found that the amount of dirt that a box can haul is inversley proportionately to the depth of the cut being made to fill the box. In other words, prior to mounting gauge wheels on all of my attachments, my BB would fill fast and dig deeper as I went and I could only get half a box or so cuz it kept wanting to dig deeper. After gauge wheels, and setting the height properly, I can not only drag a full 4' box but can watch it spill out over the top ... pretty much as long as I want. The point: A 48", 430# box, under any conditions, is fine for a BX ... 4WD, Lo range, maybe diff locked. ... a 4' box with gauge wheels is a new world just waiting to be explored.
Cheers!