Hi Rocket man,
I also found a lot of conflicting information when I was re-doing the bed of my '47. Even the kits available will have different numbers of blocks and spacers. There are six mounting points on the 3104. You will need six rubber pads (spacers) and four wooden blocks. My '47 has three fixed cross sills and one floating cross sill that is notched for the under-bed gas tank. The front and middle cross sills will use the wooden blocks and rubber spacers. The rear cross has a 'steel block' so you'll need rubber spacers in this position. All three mounting points should be the same overall measurement from the frame rail to the underside of the bed, in order for the bed to sit level on the frame.
I've come across information that indicated there are four fixed cross sills on the 3104; however, many trusted SBs indicated this was incorrect. Check out the Making a Stovebolt Bed forum
for additional information.