Dear David,
I'm just seeing your post now--as Alvin mentioned (thanks for the kind words, Alvin!), I have a 1938 3/4-ton panel on which I did an off-frame restoration, so I think I can help. In reality, the actual wood floor of the panel does NOT support the weight of the panel body, though there are contact points which do support the weight. It is assembled exactly like the wood bed of the 1/2-ton pickup (I just re-did my 37, and was reminded of it). As you know, the body of the panel at its mounting points rests on wooden blocks that sit on rubber pads (approx. 1/4") that sit on the frame. You can (and should) mount your body on the frame at those points before installing the floor.
The floor itself consists of several wooden boards (7, if I remember correctly) that vary slightly in width (the outside boards are wider than the interior ones). You can ascertain exact measurements by checking the rear cross member (that attaches to both sides of the panels at the back of the truck and sits right below the bottom of the doors. There you will see a ledge 3/4" deep, with a series of holes drilled. Your bed boards sit BETWEEN those holes (not over them). The bed boards extend from that ledge up to back of the platform where your seats sit. There should be corresponding holes in that floor panel, that line up with the holes in that rear ledge (you can pull strings between these holes that will indicate the width of your bed boards). That distance will determine the length of your bed boards. If I recall (and I'm going by memory) the width of the interior bedboards is approx. 7 1/4" (you need to leave a 1/4" gap between bed boards to accommodate the width of your 1/4" carriage bolts that will attach the bed strips to the boards). The 2 outermost boards, on left and right, are wider and have to be cut to fit around the wheel wells on the interior of the panel body. These boards on their outer edges will slip UNDER the flange attached to the walls on each side of the panel (just like the outermost bed strips on a pickup bed).
In addition to the front and rear attaching points for the boards (as outlined above), there are 2 cross pieces that sit across the frame. These look like U-channels and have a series of holes going across them (these holes are to attach the bed strips). These cross pieces sit on the frame rails of the panel, right and left, with a wooden block and rubber pad (just like the attaching points). Long carriage bolts on either end go eventually through the bed board, the U-channel, the wooden block, the rubber pad, and the frame rail. The wooden block and rubber pad should bring them up to a level that is just enough to accommodate your 3/4" thick bed boards.
The bed boards themselves have a rabbit cut into them along their edges, approx. 1/4" wide and about 1/16" deep, to accept the height of the metal bed strips.Once you have all the boards cut to length, etc., you should lay them in the bed with the holes front and rear BETWEEN the boards. Your bed strips then lie over those gaps, sitting in the rabbits and the bed bolts (which are 1/4" carriage bolts) go through the bed strips, between the bed boards, and on the underside are finished off with fender washer, lock washer and nut. Where the 2 cross piece braces go across, your bolts will go through the bed strip, between the bed boards, through the U-channel, then fastened with washers and nuts. You are right that where the boards go across main mounting points, there is a recessed circle in the bed wood with offset hole to accept a washer and carriage bolt that goes all the way through to the frame rails. If you don't have a lift, you are about to spend a lot of time on your back--don't ask how I know!?!?!?
Toward the front of the right side of the bed, you will have a round plate (approx 4-5 inches) that also needs inserted into the bed board--this is for access to the sending unit on top of your gas tank, which should be directly below.
N.B. if you don't have the cross pieces mentioned above, they are available from Jim Carter. I had my originals, so the spacing for the holes was perfect for my bed; if you go with replacements, you may have to drill these yourself. Also the bed strips from suppliers come pre-drilled or you can drill them yourself. I would imagine that the bed strips for the 1/2-ton panel are the same as for the pickup, but I'm not sure. Since mine was a 3/4-ton, I ordered the longer bed strips without holes, and I cut them and drilled them to fit my panel. If you drill your own holes, you will have to file the edges of the holes to make them square to hold the carriage bolts. It's a bit like a Chinese puzzle since all these holes must line up, on bed strips, cross pieces, and frame--but somehow in the end, they do!
That's a lot of words--I hope they are clear and make sense. If you have questions, let me know.
Good luck!
Jim
