Any reason why you chose that style bracket vs. that larger one on the NOS example? Was that a later production bracket that is on the NOS piece?
The 1969-71 Assembly Line Mufflers mainly used the smaller bracket like the ones in the picture below. The NOS version I showed in the photos was a 1973 dated unit. All Mufflers manufactured after 1972 used the wider bracket. I talked with a designer at Arvin years ago and he explained the reason for the differences. When the early version first came out there were issues with the weight of the Muffler and vibrations causing the spot welds to break loose. As a brief remedy, they welded addition supports to the small bracket (anchored to the Body of the Muffler) for additional strength & rigidity. Those changes were cost prohibitive and added too much time to the manufacturing process. They finally solved the problem in 1972 by using a wider bracket. The weight of the Muffler was spread out over a larger surface area which allowed the additional support required.
Since E Bodies were a "new" model vehicle for 1970, it took a couple of years for most Exhaust Systems to start corroding and wearing out. There was not an abundance of those units manufactured for service replacements during that time. When the demand for Service Muffler's increased, the new larger style brackets were being manufactured in full swing and also used to service the 1970 E Body vehicles. (Warranty Claims, etc....) Even though the brackets were visibly different, they could be used in place of the old units that originally had the smaller brackets. The dimensions and mounting points were the same. This is why the early style Assembly Line/NOS Mufflers are much rarer and more difficult to find than the later styles. The early styles were not manufactured as long and the later style was used as their replacement. Below are photos showing the 3 original styles discussed here.