[RockHQ]

Here you can find a dyno run done by V8-Magazine Finland back in '95. Doesn't list many mufflers but i thought it might be an interesting read. You can see the difference between various mufflers is minimal with a slightly tricked 454, so i'd say it will be even less with a 350 or a 305.
Another interesting tidbit about backpressure: compared to straight pipes (minimal backpressure), the best muffler provides an average of 0.7% more torque.