I have these;
Patriot Exhaust H3200 - Patriot VaraFlow Tunable Mufflers - Overview - SummitRacing.com
Expensive, but I like them. Don't know if they could be adapted to a cobra. Maybe. 2 1/2" is a little small for a big block, but it's the only size available.
Essentially a glass-pack with an electric flapper valve.
It just so happens, when at my first track day earlier this month, I got a noise violation warning (exhaust open.) It was incredibly loud, while pushing the engine hard. Much louder than the small block cobras, GT40 replicas, and the new roush mustangs.
Problem is, when you close the valve, it does knock off the engine power. I was about 10-20 mph slower on the long straight. Bad for trying to catch 80%+ of the cars who were supercharged.
Video, with the exhaust closed - as you will hear, pretty quiet, for an FE.
Thunder 1967 Shelby Roush Track Day Aug 8 2011 Driving Instruction - YouTube