Scott,
Way back when there was a bunch of articles about doing this.
I cannot remember the magazine but a search on the internet led me to this site:
http://webrodder.com/articles?search=six+pack+to+goThe one you want to look at for this job is:
http://webrodder.com/article/six-pack-to-go-part-6-converting-a-late-12-bolt-for-trailing-armI can't remember the exact date but I did this exact thing probably 15 years ago (I had the actual magazine it was printed in).
If you do not want to mess with your original rear end I guess you could get a spare to work on.
Bottom line, I do not know of anyone who sells this bracket. Maybe you could get 4-5-6 Chevy to get you one but you would have to contact him personally as I do not know.