How about connecting the motor directly to the transfer box. Gearing may need to be changed and you will still have high low and even more space for batteries under the bonnet.
Deep cycle batteries are easily accommodated in a 90 with the configuration above. I did some quick sketches of a design a few months back using and Advanced 9" motor and at 144v. I found space for 24 Yellow Top Optima's Below the floor (TD5 tank position, in the wheel wells, 4 in the seat boxes and the rest under the bonnet. Still leaving loads of space on top for the controller.