I have a 1971 Chevelle with a 350 / 350 auto. The engine is mostly stock (rebuilt), with the exception of headers, dual exhaust with flowmaster 40s, an MSD 6AL box with HEI distributor (had to get rid of the points), and a mild RV cam.
Inside the car, the 40s are surprisingly mellow -- I can easily talk to passengers or on the cell phone, even with the windows down.
Outside, the car sounds great -- I couldn't be happier. It's loud enough to be noticed, especially when I get on it, but quite enough that it doesn't disturb the neighbors when I leave for work.
Personally, I couldn't have asked for a better sound. To boot, I paid $200 total for two 40s with installation.