Gus,
there is a 6" round Ultra Flow that is 30" long, part # 17299, 3" I/O. I'm not sure if that case is too large in diameter.

In oval mufflers there is an Ultra Flow welded muffler, part #17235, 24" long case, 3" I/O side/center. Body is 4-1/4" x 9-3/4".

That's all I could find in my Dynomax catalog.

Brian - 1971 Dodge Challenger
"The Dodge, which was practically stock, just left the Mustang like you wouldn't believe."