[euromodder]

The front end is where most efficiency is to be gained.
A Partial belly pan at the back can lead to parachuting (catching wind underneath it) and means the wind would be dragging at the belly pan.

You could leave a slit between 2 parts of the belly pan to extract the heat from the engine/
(aft part being mounted somewhat higher than the front part)

At the rear axle, you could do something similar, splitting the pan @ the axle and flexibly mounting both parts to the axle so they can move up and down with it. Remember to give the belly pan parts some play to move fore and aft as the axle moves vertically.
(front pan once again being mounted lower than the rear one so the wind won't catch the rear)