A few years ago I picked up a set of IMPCO oe style replacement mufflers. The were the correct oval style, offset 2.5 inlet and center 2.5" outlet. There weren't a lot, $35-$40 each. Have them on a 500ci car and 5.7L car and they're quiet. They were a little longer than stock B-body mufflers but they fit. You might be able to google the company catalog and find them. I'll look for receipts and try to come up with part numbers.