FWIW, I started taking my Triumphs to a local muffler shop and having them do it. The cost is minimal (sometimes even less than the parts would have cost me) and the work is good. Best of all, I don't have to crawl under the car and fight with it, while the rust rains down in my eyes! When I converted the Stag to OD (which believe it or not requires a different exhaust), he modified the existing headpipes to fit, for free! Saved a big wad of money there (as pre-bent pipes are only available from Merry Old England).
He offers a variety of mufflers, but I always opt for the straight-through "Cherry Bomb" style glasspacks in seamless aluminized steel. They seem to last nearly forever and are a good match for the original mufflers (for TR3 and Stag any, not sure about TR6).
Anyway, just a thought. You might want to check with a local shop before buying from BPNW, etc.