Both flowmasters and glasspacks have different sound levels,
With Glasspacks, shorter is louder! A 40inch glasspack is very quiet, my friend Mel has them on his '49 Chevy, and they are really quiet. Get the shortest possible, or not run them at all!
Flowmasters, sound output depends on the model, the original 40's are the loudest of the group. Im thinking the 70's or 80's Deltaflow are the quietest, but they still have that Flowmster sound.
Fly-by-wire cutouts would be a great idea, have them switched to a quiet Flow' or 'pack, then when you want to REALLY rip it up, switch it to straight-thru.