My local Norton guy sends the heads out to a friend of his who welds in fresh aluminum alloy and cuts new threads.
Wes Scott in Fort Lauderdale. PM me if you need his number.
He says this is a common problem on 850s because of the re-design of the exhaust to accommodate the balance tube. The 850 exhaust nut doesn't go into the head as far as the 750, so the load of the exhaust is carried by fewer threads, causing undue stress, and making it easier to strip the head.
He's a fan of non-balance tube exhausts for the Norton.