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A friend of mine asked me this and I did not know. He has a 67 427 with side pipes and the stock covers. The motor is a 550 hp motor on thedyno. He is currently running manifolds and the stock side pipe exhaust. He is looking to go headers and run bigger mufflers in the stock covers. he wants the stock look. The exhaust is chocking the car now and he wants it to breathe better. Is there a larger or better muffler that will fit in the stock covers. Slight modification to the cover is not a problem he just wants the stock look.
You're right about the stock side pipes being restrictive. About 75 HP worth! For performance reasons, a person would be better off with the "off road" exhaust.
Stainless Steel Works (I believe in Ohio) make and sell side exhaust systems for mid-year Corvettes. On their website, the 2 1/2" dia. sells for $730 in S/S and $430 for aluminized. Look under the muffler section, all the way at the bottom under Corvettes. Their web address is: http://www.stainlessworks.net