I'd be tempted to use an idler roler on the belts to get the tension right. Its fast and easy
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Id also use slipper pulleys front and rear, either on the spur shaft or on the gearboxes.
The reason I thought about using 3 gears in the boxes is to get the height to clear the lipos. Using all B4 internals would cut down on costs and there is a gear or ball diff option there too. I'd make the gearboxes split left to right but also have a seperate diff cover so a few screws will get the diff out easy.