I used a 5' on my B7800 (30hp) and it handled just fine. I think that was a good match for the tractor. If you are mowing a field with saplings or very tall heavy grass just take less then full cut and go slow.
Remember that not all five foot brush cutters weigh the same. A true heavy-duty cutter may truly weigh too much for the B machines. That said, I've used a five foot Razorback on my B7800 for a couple of years without trouble.