Originally Posted by Jersey Mike
Don't get me wrong, your car will still be noticeably louder than the legal limit, but it will not be AS loud as it is currently (Flowmaster are generally on the louder side of the muffler spectrum).
The Magnaflow will quiet it down *some,* but don't forget that you're running a cam, long tube headers, and no cats, through a V8.
Below, I've broken down why your exhaust is as loud as it is, in order of significance from greatest to least:
Camshaft: the main culprit in your struggle with volume. However, changing this would require at least 6-7 hours of time (yours or a shops) and would decrease your performance by a noticeable amount. Your car would significantly quiet down, though, by switching it back to a stock, or less noticeable cam.
Headers: The second biggest player in this issue. Doing headers is usually a 3-4 hour job and, as again, you're going to be losing something like 15rwhp (which, isn't as much as you would lose by changing back to a stock cam, but still a significant loss).
Muffler: This is the third largest thing to come into play. You're currently running a Flowmaster, which isn't the greatest, but is loud. It's sole purpose is to be loud. There's no performance to be gained. Swapping to a more quiet muffler, like the ones suggested in this thread, will quiet down your exhaust some. Due to the engineering (or lack there of) that goes into Flowmaster R&D, you'll actually pick up some power by switching mufflers. Win-win.
Cats: You're not running any catalytic converters, right now (Hence the "off road" title, as Cats are a federal regulation). In all honesty, they won't quiet you down much at all. They will remove some rasp, but don't expect your exhaust volume to suddenly change all that dramatically. If you don't race this car often, then I'd suggest looking into some high-flow cats at some point in the future. I have the Thunderbolt Spun Metallic cats, and at $55 a piece, they've been great for both passing inspection and eliminating a little bit of rasp compared to my previous ORY set-up.
In your case, don't touch the motor or the headers. It's not only a shame to get rid of them, but it will cost you a lot in labor as well as require a re-tune. Just pick up a nice muffler (I think the used one I linked you to was only $100 or something, versus the $350-500 that the new ones go for) and work your way towards the front if it's still unbearable after the swap.
If your problem is interior volume, there's always sound-deadening option, by the way. But, that doesn't help if your main concern is with the police in your area.
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