Epilogue on this thread.
I replaced the Dynomax muffler with Pypes Turbo mufflers and 90% of the annoying resonance is gone. I still get a little at a very specific RPM but all in all I am happy with the result.
Thanks to all those smart and experienced people who commented.
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1971 GTO Convertible 455 #64 Heads, HO intake and carb with MSD HEI, RAIII exhaust manifolds and 3:55 posi
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