The classic mustang crowd (200-250ci I-6's) seem to have success with headers, Y pipe if dual headers to 3" pipe, single in dual out flowmaster 40, dual 2.25/2.5" exhaust out the back. Especially with a performance cam, this sounds like a muffled v-8, very nice, not sure how they do it, when reved it doesn't rasp like a six normally does, IMO thats nice. I have heard a flowmaster 40 with a simillar set up on a larger ford 300 I-6 and it sounded like an I-6 with glasspacks...weird..I would think the sound would be deeper and mimic the smaller I-6 but apparently not.
The difference is probably in the length of the primary tubes on the headers. My 6 has the same sound as you describe of the smaller Ford 200-250 sixes - very v8-like. And when you open it up it retains that low note, instead of raspying up like true duals on inline 6s tend to do. I love my sound and would have it no other way.
-magic mike-
