Well, there's this, but it's a shorty, has a 2.5" option, though:
http://www.lossaengineering.com/shop/lossa-brand-reverse-cone-muffler/
Other than that, I'm not finding anything, either. I don't think the pipes on my '79 750 are that big - maybe the got away from them after '77. If you end up going an adapter route, i recommend the EMGO pipes - inexpensive, sound good and look good and they make a Dunstall replica (it's only 1 3/4" though).
good luck!
GMK