I installed a pair of these "resonators":
https://www.dynomax.com/mufflers/rac...und-24405.html
These are on the tailpipes of my '66 GTO with full 3" exhaust and Super Turbo 16" or 20" mufflers ahead of the axle (forgot which).
https://www.dynomax.com/mufflers/sup...set-17925.html
The bullet mufflers don't look like much but they work surprisingly well in addition to the normal reverse-flow mufflers. Being 3", the system is still loud, but now more tolerable. Engine is a 250-ish duration solid-roller 428 with 72cc E-heads.