Magnaflow makes a complete kit for '64-72 GM Abodies. These came with it. They are 2.5" offset in/center out and really short. When I opened the kit and saw how small they were I figured the car would be rather loud. I wasn't looking for super loud, but wanted a nice throaty sound. I installed the system (fits the car really nice), but didn't fire the car for another 6-8 months. I was rather let down by the sound and have been unhappy since. I think the X pipe makes it sound odd too.
The main reason I went with Magnaflow was because of full stainless and the mufflers were polished (the entire undercarriage is detailed) and I wanted it to look good for a long time. At the time Flowmaster did not have a complete setup in stainless (my 454SS has a complete mandrel bent, fabricated exhaust and I wanted an out of the box system for the GTO). I have always been a Flowmaster fan. I have 3" flowmasters on my 454SS, and ran them on many cars/trucks over the years. The 454SS sounds pretty bad ass and also has an X pipe, but also has a whole lot more motor. My 5.7L Vortec K2500 I just did a Dynomax UltraFlo daul 3" in/out with 3" tailpipes and 4" tips at the back. It sounds pretty good, but a little too loud for me. Everybody comments on how good it sounds.
I have heard dead stock GTOs with gardner reproduction 'stock' mufflers that have sounded way better than my car sounds. I have never really heard anything else with Magnaflows.
Here is a shot of the underneath of my GTO. It still looks about the same after 6k miles. It's a ***** to keep this clean though.
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Oh and BTW I never heard about the lack of drone when standing the mufflers on end. My 454SS they are stood up and there is no drone at all. My K2500 has all types of drone and the muffler is lying flat.