I have 3" w/dumps on the mustang. The larger the pipe, the deeper the sound. I really like the sound of the bigger pipe... and since you're running dumps anyhow, you may as well use the more freely flowing large diameter pipe.
The 347 will definitely breath more freely, and should pick up significant power.
Just try it... You'll like it. It WILL make more power. If it doesn't pick up much, it will surely sound like it did!