The welded Flowmaster somethings came off my 76 New Yorker the month I got the car.
Went with Walker Quiet something's. Very happy. No drown. Two cats, two mufflers now.
Power loss? Stock 440, heavy car, who knows.
Power? it doesn't really matter unless you are class racing. ET racing is a dial in. Street performance is limited to how much bite is at the stop light and 300 horse is about enough.