On my old b6000 i fabricated this subframe to be used with a genuine kubota loader, it worked well and was more than upto the job of a subframe, also came apart into smaller section to make it a bit more user freindly to fit and work with.
key things with subframes is to remember that they need to be well engineered, don't be tempted to skimp on materials or use undersize sections, the subframe performs a critical function in that it not only provides strenght for the loader but also provideds a secure mounting point for the loader and ties tractor to loader into one unit, all the stresses and strains your loader takes is absorbed into that subframe also transmitted into the tractor chassis etc, better to be over engineered than under.
there are a lot of pics etc on here from myself, maybe worth looking back through some of my old threads.
anyway , here is a starting point.
also some details at this link, a few years old now, but may be of some use, as i say , i've put afair bit on the forum you'll find more than enough by using the search option.
anyway link here:
http://www.orangetractortalks.com/forums/showthread.php?t=1873&highlight=b6000+loader
rgds
rob