I have been driving my 62 390 since Nov 1969 . I have always had stock mufflers and resonators and I enjoy the fact that it is very quiet . In 2000 I put a new dual exhaust system on it and again it was very quiet.
Recently I repaired the intake manifold as the choke tube was leaking so i ordered the Olson block off Gaskets. When I started it up the exhaust note had become much more aggresive. In short I hated it !
I tore it down again used stock gaskets and the sound returned to normal. I have a friend with a 63 that did the same using Olson gaskets with a single exhaust and it was much less aggressive sounding and we both agreed it was not a problem. We both used spacers in place of the heat riser.
I assume it has something to do with a crossover pipe when using dual exhaust. The intake exhaust path is serving the purpose of a crossover pipe. With single exhaust, both sides are routed together so a crossover pipe would make no difference. Does your dual setup have an added crossover pipe?