I have the same exhaust system on my 74 Charger with a mild 440. I started with Spin Tech mufflers and the drone was a PIA. I made some inserts for the tail pipe that made it better, but was not the cure. I switched to Dyno Max and the drone was reduced.

2 most common causes for the dreaded drone:
1- The muffler resonates due to a free flowing design and the muffler casing vibrates like a speaker.
2- Tail pipe diameter is the same diameter as the pipes in front of the muffler. The exhaust cools somewhat and the tail pipes can become something like a megaphone. If the tail pipes are 1/4" smaller, drone decreases.