This is the basic layout I'm planning for the BJ₂O (yup, named it already):
The front sub-chassis could be moved rearwards if the long wheelbase (which requires minimal cutting of the chassis) proves unsuccessful.
A quick, unscientific test of balance showed it was about right - motor + mount balances approx 2/3 of the lipos, so 200g v 200g roughly.
Receiver would go in front of the motor (I use Assan receivers, which are very lightweight) and the ESC would go up front and centre, behind where the servo would be (which might require the wheelbase to stay long!). It can be subtly shifted left or right to fine-tune balance.
Bracing the chassis might simply require long rods from the sub-chassis to the motor mount. Going down the sides is made tricky by the lipo position. I'd also consider making a removable piece for bracing the motor mount to the rear gearbox, to allow easier slipper access. I can cannibalise a BJ4-WE rear top-deck for those bits.
Alternatively, I can simply employ a BJ4 original top deck - the front central hole on the sub-chassis lines up with the front centre hole on that topdeck.
My major drawbacks are lack of machining skills or tools, and time...
This layout could work on a Kyosho ZX-5 (not FS) and a few other shaft-driven, non-mid motor 4wd buggies.