3-4 feet square underneath the transfer case (external rot trapped by
reinforcement)
both rear corners round the drain valves,
prop tunnel reinforcement (angle iron) mounts,
all round the rear axle pivot mounting points,
front wings internally at joint with marine ply directly above wheels,
All these areas needed work on mine, cost 5000 pounds to get it right.
In your situation the "easy" solution is to drain all the fluids and
throw it up on it's side, then sandblast the whole of the underside,
cut out LARGE areas of sheet metal (dont bother with ten small repairs
when one large one will be more satisfactory - underneath the transfer
case is a good example) The joy of doing it this way is that your
fabrication costs are limited to getting side / floor reinforcement
pressed up, and be generous with the quantity as you can always use the
extra or sell it. Apart from the reinforcement everything else is
plain sheet of varying gauge and the trick is to cut back big enough
areas to get to solid metal. If the rear axle mounts are poor it is
quite straightforward (I'm told) to drop the bogie first, and this
makes access to the rear floor much simpler. Check your driveshaft
tunnels and seals too while you are at it, and give the pillow block
above the intermediate axle a good check the lubricate it to death.
Most DUKW road failures are pillow block / rear axle diff / transfer
case related , if in doubt disconnect a shaft or pull the halfshafts
from a suspect axle, and when fitting tyres try to get a set of four
with even wear amounts on the back. Even in perfect condition a good
DUKW will make more noise than a standard 6x6, but watch out for that
pillow block.
Gordon 8-)