As promised here are some pics of the inner brackets I made for the '72 rear bumper swap.
Pic 1 - I cut off the tubes of the bumper shocks and reused the mount ends to utilize the existing hardware and reinforcements that are built into the rear frame horns. Then salvaged some 1/8" angle to make extensions and some left over tabs from the Tri 4 link with welded on nuts for the inner bumper bolts.
Pic 2 - Bracket set for the '72 bumper. There are small outer brackets from the stock '72 set that I didn't use. The red arrow highlights the outer bolt location that has to be drilled thru the lower tail panel. Blue arrow for the inner bolt to the new frame brackets.
Pic 3 - This is the hole for the outer bolt that has to be drilled. You can see the oblong hole in the inner panel that is/was the location for the stock hole for early 70s rear bumpers. Thankfully the factory didn't change the inner panel stampings used on '73 and latter cars so it makes it very easy to convert.
Pic 4 - New outer bolt hole (red) and inner mount (blue) from the outside. You can see the outline and hole for the old bumper shocks and how the new bracket is situated.