I have 3" stainless steel Magnaflows on my modded 383. Back when I was getting ready to order the exhaust system (tti 1-7/8" headers & 3" tti pipes) I did a lot of research on mufflers. I found that Magnaflow and Dynomax Ultraflow mufflers outflowed nearly all the performance mufflers on the market (including Flowmasters) by a pretty wide margin. I believe Mopar Muscle did a tech article in which they compared Dynomax Hemi Super Turbo mufflers against the Ultraflow mufflers using a brawny 440 in a 67 Coronet R/T. They found that by just switching from the Hemi Super Turbos to the Ultraflows the car picked up .15 in the 1/4 mile. The test was conducted all in one day and the Hemi Super Turbos were tested early in the day when the air is better.
The Ultraflows and Magnaflows are basically the same design - a straight through perforated pipe wrapped in 2 different kinds of steel mesh (think steel wool) encased in a housing. They are a few decibels louder than other mufflers but the results speak for themselves.
Lately, the Hooker Aero Chamber mufflers have been showing great flow potential as well. As good as the Flowmasters were when they came out, there are much better choices to be had today. Glasspacks are old hat and won't last all that long. The Cherry Bomb Turbo mufflers are ok - I had them on my '71 Charger SE and they sounded good but they reached a resonance at about 40 mph that would just about drive you out of the car. There are lots of good quality offerings out there today for general street use so I'd avoid the glasspacks altogether.
For general street use Flowmaster, Dynomax, Hooker and a few others make some good mufflers that will flow ok. If you are going to play with it on the strip and want a good flowing muffler that will allow you to run the car through the exhaust system without dropping the head pipes (on header equipped cars) you should seriously consider the Hooker Aero Chambers, Ultraflows or Magnaflows.