I did similar on my classic i lay underneath it made a angle template.Went to a tyre exhaust center and asked them if i could have a bit of scrap exhaust pipe..Help yourself was the answer,I found a really nice section with the bends just where i needed them,I think it was of a merc sprinter van.Went home.Next day Removed the rear section from the classic, Cut end off original pipe,The scrap pipe i tack it on to the original that i had removed and cut.Offered the modified end pipe up all looked ok.Marked and cut hole in lower panel.All now looked ok ,Removed the pipe again and mig welded it.Nice chrome end piece and the job looked a good un.