When I think classic musclecar sound I think Thrush & Cherry Bomb. Flowmaster and such IMHO is a modern muscle sound. I had 2 baked out long thrush glasspacks (true duals) on my 73 Plymouth Roadrunner w/a hopped up 340 and that thing sounded beast and as classic as it gets. I plan on putting a Thrush welded muffler on my 1990 c1500 350 w/a high flow cat soon.
Ummm a Thrush "welded" muffler is a duplicate copy of a flowmaster 40. According to you, this is not a muscle car sound. I have a muscle car with completely cooked glasspacks on it.
Magnaflows are damn good, I had an itty bitty one mounted up on my old Vortec 350 Suburban, all my friends said it sounded like a racecar when I got on it, still had stock manifolds and cats too;
My 454's got a flowmaster on it, sounds decent, but just want something better flowing. Plan on running shorty headers, no cats, true dual 2.25 pipes with an X-pipe and maybe an H-pipe welding in somewhere.
Another good way to achieve the muscle car choppy sound is a cam upgrade, but that's a whole 'nother can of worms that a lot of us are way to eager to open.
Sound is one thing. Rhythm and beat are another. Come to think of it, there are quite a few distinctly different muscle car sounds.