You need grounds from each alt to the front battery.
What size/brand wire? 2/0?
How well you protect, route, and secure wiring will make all the difference for safety. No idea how you will land that many wires on the front battery safely.
I'd take each alt straight to the back buss bar, minimum 2/0 power and ground on each. Then back feed the front battery through an isolator. It's just for starting and vehicle electronics anyway, correct?
Rear battery to chassis is for safety and voltage stability, so a fault will actually clear. While the chassis isn't being relied on for a current path, it will exist since the alts are bonded to the engine which is bonded to the chassis. Upgrade all current paths, power and ground, for full rated ampacity.
Helluva build.