[sanddrag]

The best way to do this is have the Dewalt gearboxes direct drive your center wheel axles by means of flexible coupling (I recommend aluminum 3 jaw spider couplings made my Lovejoy and sold through Applied Industrial). Then just run a chain from the center to the front, and from the center to the back, on each side. If you have to, you can really drive any wheel from the Dewalt, but the centers are preferable because they are always in contact with the ground (if the center wheels are offset relatively lower than the rest). If you have to space to do it, you should avoid having 6 chains. It is more potential failure points.

EDIT: I missed the "total of four" thing