there's at least 3 designs of 'mufflers'- chambered, turbo, glass pack...
I think the 'chambered' style is known 4 that (flowmaster) drone. Not too pleasant esp on a long trip.
As per bubba - try the turbo (I think it's a tube inside there, w/holes along the tube) they're reportedly better for that prob (and performance in many ford6 opinions). Our bud, Matt, at vintage inlines dot com can assist w/a purchase frm his stock. U could even put a resonator in there B4 the muffler to help the flowmaster...
U can have the 2 pipes go down the body & into a muffler or have a muffler for each side (1 or 2, ur choice).
DynoMax, Borla, Thrush have turbo style...however - many co.s (even these I believe) make all 3 varieties so double ck.
google for the 3 or 4 types for more info).
Good Luck & let us know how U do!