imperium, I've got a 40 series Flowmaster on my 4cyl HB w/ a K&N filter and it sounds pretty throaty, specially when downshifting into 2nd. Maybe contact Dynomax, they used to make a cat-back system for the HBs. A 40 series Flowmater might be a little too loud on a 6 cyl, but then again it might just be what you are looking for. Just some options.
I have a 40 series Flowmaster on mine. Doesn't roar like a stang, obviously, but def has a beefy sound. I have to warn you that the drone is not soft...but I usually have my radio on pretty loud. I want to say the model was 4140 or something like that. I only did 2 in inlet/outlet as a 6 cyl NA motor doesn't need anything larger. could mess with the back pressure