Wow. I thought the 6400 was an unsupported system by virtue of it being a strange, PPC/Nubus/non-OF system. Clearly I was wrong. ;->
I think you're thinking if machines like the 6260 (or 62something). The 6400 is a simple PowerPC 603e, a couple of DIMM sockets, a PCI riser and a comm slot.
John