Muffler selection is a matter of personal taste, but I've always liked the Walker Dynomax turbo mufflers. Nice balance of sound and low restriction without being obnoxious. They also have been proven to outflow Flowmasters.
For pipes, 2.5" should be fine. An X-pipe (as opposed to an H-pipe) will improve flow and reduce noise by allowing every exhaust pulse to see both mufflers. Several vendors sell 2.5" prebent pipe sets for A-body cars, but Pypes Exhaust (
www.pypesexhaust.com) sells complete sets with X-pipes AND 2.5" trumpets for 442s. The stock tailpipes and trumpets are only 2".