I believe a 1/2 ton would have the following cross sills - one attached to the front of the bed, one forward of the frame hump, another behind the hump and then the rear cross-sill.
I think that the wood blocks and rubber pads should put the bed parallel with the frame (the part forward of the hump.) If your filler neck fits in the bedside hole, you're probably good.
Moving your post over to "Making a Stovebolt Bed" where it'll be more on-topic.
