I used 2X4" square tubing attached to 1 1/2 X 3 formed closed C-Channel cross sills. The perimeter is welded in at 3X4' above the sills to allow for the wood floor to sit level with the top of the outter frame. It's a little heavy but that's as small as I could go to still be able to use the truck as I designed it. I want to be able to carry my harley's if the need arises. Anyway here are a cpl of pics showing the proccess.
Cross sills are held in place with 3/8th bolts, lock washers and nylock nuts.
Then I cut the 2X4 to the right sized and welded it to the perimeter of the frame. I cut around the wheels to keep it as low as possible.
I still have a little welding to do today like weld in the caps for the open ends on the 2X4s. I'll order up the wheel wells/tubs on Monday, once I get those in I'll decide on the wood for the floor and side gates. Also I still need to source the tailights I want to inset into the tubbing. All in all a good productive saturday.