ACE
look at Classic Cobra Pack mufflers 3" with the louvers turned to the engine. Measure your old muffler and see if you can install longer Cobra Packs.
When you install longer mufflers, they are quieter but allow more exhaust out.
My 354 / 302 stroker put made over 400 rwh - tq and I think a lot of it was my exhaust system.
Classic Chambered Exhaust Inc. - Cobra