You may want to take a look at the following thread for the details of the job, and check with Bob at Accessoryland. He actually had a couple of outside and inside brackets available a few months ago.
http://67-72chevytrucks.com/vboard/s...d.php?t=401449
The outside brackets are essential to the install. If the inside brackets are unavailable, the standard clamp type will work just fine as shown in the thread above. His website use to come up on a Google search but I'm not finding it now. I find two phone numbers listed: 563.556.5482 and 319.556.5482. (Perhaps they had a recent area code split).
It's a mom and pop outfit so he's a little slow to respond sometimes, but has always come through for me. I believe he is the only antique parts dealer with open access to the Unity warehouse and inventory.