What mufflers for 340

If muffler CFM is your concern, a straight through perforated pipe muffler is where you should be looking. Pypes Race Pro, Magnaflow, and Dynomax Ultra Flo come to mind. The Dynomax Ultra Flo's will handle up to 2,000 HP. It's the muffler of choice for the F.A.S.T. Drag racers. Dynomax also has their Super Turbo muffler which will handle 700 HP and really control sound and drone. Any of the above straight-thru mufflers will give you decent sound levels at idle and cruising, but bark under throttle.
Pypes - Classic Muscle Car Exhaust