Not that any of this is really necessary detail for the story, but just because it's fun:
Redundant organs are more expensive and probably less effective (a second heart still bleeds enough to kill you) than just putting all the important organs in the back.
Front: intestines, bladder, pancreas, liver, etc. stuff that keeps it alive day to day but isn't needed immediately to finish an attack charge, and a metric **** ton of exoskeleton and cartilege. Ideally interleaving layers of skeleton and cartilege but I don't know if we know enough to do that.
Back: heart, lungs, most of the central nervous system, highly bleeding things like kidneys, sensory organs*
*Eyes would need to be in the front, everything else can go in back. Eyes aren't terribly mission critical, though, as it's big enough to crush small roots and things and won't trip, and the hive mind can tell it which general direction to go, since it will be escorted in battle by all the infantry anyway. Eyes are also small and cheap to grow, and you could just have 20 of them to allow it to continue seeing in a hail of gunfire, too.
Body shape: roughly the shape of an Avatar from Eve:
But, you know, with (several) short legs, and a little shorter in the back. And the frontal plate would be cut off a bit on the bottom to allow ground clearance.