I put a Flowmaster American Thunder dual exhaust kit (40 series)on my 68 w/ headers, 2.5" pipe, etc. sounds great in town - but too loud on the interstate - can't talk on the phone, hear the radio, etc. I was told (by Flowmaster) to try the 50 series, but when we took them to the muffler shop and put it up on the rack, these were just no going to fit without making MAJOR changes to the whole exhaust pipe system, so Summit suggested Walker Dynomax Super Turbo - same exact size muffler, but much more quiet (according to Summit). They did say I would have a little loss of horsepower. Any opinions on this muff swap? Has anyone else been down this road? I have tried B-quiet already - it helped a little, but still pretty loud on the interstate. I was also considering an 200R overdrive tranny - which would lower the interstate rpms and improve gas mileage - thanks in advance for youyr opinions and help!

We used to have a 331" Corvette vintage road racer that I put Super Turbos on and it didn't slow down at all. We dynoed it at 380 hp to the rear wheels at 6000 rpm.

Yeah, the Super Turbos are a good choice, nice and quiet until you open it up, and even then they are much quieter than the Flows.

I don't think you'd notice any loss of power.

This is EXACTLY what I did, I had dual Flowmaster 40s dumping before my rear end, I changed out to dual Dynomax SuperTurbos and Flowmaster Mandrel Bent Exhaust Tips to exit the rear, now I can have a conversation in the car or actually think, yet its still a good sounding vehicle if you really step on it! I did not notice any power loss.
Exhaust Upgrade Details (http://69camaro.nextmill.net/exhaust.htm)
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http://pontiacstreetperformance.com/psp/exhaust.html