Stop putting Giant oversized pipes on these cars! Bigger is not better. i must get two or three calls a week over someone looking to buy a 2.5 or 3 inch exhaust kit for their nailhead powered car. This 401 has 2.5" on it and has zero low end power. Feels like a 264 with non VP dynaflow. Exhaust systems are not a one size fits all. Everything with engine building is a trade off. In a perfect world, the exhaust diameter on the car should increase in size throughout rpm range, to be most efficient... But all we can do is pick one size to represent it's widest most efficient range, which many, many factors play into. Exhaust that's too big creates turbulence and NOT velocity. Turbulence hurts flow. You want to keep the exhaust as hot as possible for as long as possible. Exhaust that's too big, allows the exhaust to cool too quickly, causing the air to become more dense, quickly, making the exhaust "stack up" and hurts flow. Exhaust that's too big also drastically hurts scavenging. scavenging is a vacuum effect as a product of velocity, which aids the next exhaust pulse out of the cylinder easier. its a common belief that the exhaust on your car is a steady stream of gasses, when it is actually in fact a repeated series of pulses. every time this topic gets mentioned, there's people that immediately compare studies they've read on whats best for a racing application chevy engine at wide open throttle, and they mention how back pressure (being good) has been proven as a myth. take note that i did not mention back pressure a single time in this post. back pressure is a result of exhaust being too restrictive for efficient flow.. On the 322/264, 1.75" dual is ideal. On the 57 and later, 2" is best unless running headers, then go 2.25. never 2.5. switching from 2" to 2.25" on an otherwise mostly stock car will provide notable loss in low end power. take note that low end power is what these engines are designed around making.
as far as picking mufflers, most of it comes down to preference and the sound each person likes, from a street car standpoint. the era and theme of your car should also be taken into consideration. flowmasters on a 50's themed custom car or hot rod, does not work. throatiness and "rap" comes from long length after the muffler. on my cars, i typically run short glass pack very far forward, typically mounted behind the drivers seat. the bigger the exhaust pipe, the louder the exhaust. you can get away with shorter mufflers with smaller diameter exhaust. the further back you mount the muffler, the louder the exhaust is, as well, and the more "tone" that you lose. most beer can traditional style mufflers these days rust out really quickly and split at the seams , and really don't sound any different than the one piece sealed style ones, making them a better, longer lasting option.