Any modern performance muffler will perform about the same on a stock or mostly stock Boss 351, they all flow a ton of air. The only question is what you want it to sound like, and that is very subjective. Flowmaster has a bunch of different mufflers with different sound levels. Best bet for you would be to go to a car show and find some classic mustangs and see what sound you like, and ask the owners what they are running. When I was young I used to love the 2 chamber Flowmaster, but nowadays I cannot stand the loud noise and the drone. I have been running Dynomax Super Turbo's which flow well, but are not loud on some of my cars, but these may be too tame for you.