The original boards (for the 73-87) were designed to be screwed (bolted) to the rocker panels weld seam at the bottom and a couple pieces that went up behind the cab corners and bolted through the cab corner. You had to drill holes in the pinch welded part of the rocker and also a hole in the cab corner as well as under the fender well for sheet metal screws.
I picked up a set of polished aluminum diamond tread boards made for something (I think even a Ford) and made my own support brackets from 1-1/2" x 1-1/2" x 1/4" angle iron welded in a offset S (or Z) shape that bolted to the seat brackets under the cab floor, then horizontal under the steps. This was enough support with two of those, and screwed to the inner fenderwells that my fat azz could step on them and they wouldn't sag or tear up the rockers. I could have added a third support if needed. Most of the support was in the area where the stepping occurs.
If I were to do this today, I would build a set of "sliders" from 1-1/2" OD tube and make brackets to take them back to the frame. You may need to make the set split between the cab and bed to allow for the flex between the two halves.
If you have looked for these, you have not looked hard. However, you won't find them specifically for 67-72 because they didn't start until 73+
http://www.jcwhitney.com/running-boards/c1908j1s17.jcwx
http://www.autotrucktoys.com/ram/Dee...Sl8aAnL28P8HAQ