An inline six is never going to sound like a V twin no matter what you do. Also exhaust sounds are so subjective it really gets you little in discussing them unfortunately. It is something you need to hear and then ask the owner what he has if you like or even if you don't so you don't go that way with yours.
I remember the old Chevy school buses we had when I was in school (40's-50's) and they all had straight pipe completely to the rear. They had what I think is that unmistakable old GM Inliner sound.
Best of luck on this really difficult subject.
