[Gav]

Glass pack mufflers are not known for being quiet.
If you are serious about muffling down your side pipes you might want to look into building them with a spiral or baffle arrangement.
I ran 2.5" perforated pipe packed with stainless wool and then wrapped with weld blanket.
They weren't the quietest but they were easy to live with.
The weld blanket made the surface temp of the pipes lower too.