Here's the one I've been wanting to try:
http://www.summitracing.com/parts/WLK-17853/?rtype=10

It's 2 1/2" in, 2 1/4" out, 18" shell length. Lists to be a direct-fit for 68-76 Cadillacs, so it should flow pretty well if it was spec'd for that engine size.

The Walker 17000-series is their turbo-muffler product line.

$28 at Rock Auto, too!