Check with Jeremiah King or Teeitup, anyone that parts out these trucks on a regular basis. The brackets are often left behind when rears and bars are removed since they're a bit of a pain to grind the frame rivets off. Just as the HO, D60 and 12 bolt panhard bars are all different, I seem to remember their respective frame mounting brackets being different as well. Most guys missing them just run an aftermarket adjustable bar, which comes with its own upper and lower mounting brackets.
12 bolt frame bracket just for reference;
http://67-72chevytrucks.com/vboard/s...08&postcount=1