IMO flowmasters are not a good choice. They use a chambered design that is not free flowing. Yes it flows better than stock, but not as good as you could get for the same money. There are several companies that make straight through mufflers: Magnaflow, dynamax, borla, etc...
Flowmasters are often loud in the cab (drone) also from what I've read on the forum. You can go with a longer magnaflow or other straight through type and it shouldn't be too loud. I'd recommend an 18" or longer for you. Many go with the 14" and they are too loud for their taste. If you leave your stock resonators on also (near the tailpipe) it will help keep the drone noise under control too.
Breathing in and out is good enough for me.