LSA or Duration to fine tune glasspacks??

My first car was a 1967 Belvidere II convertible with a Poly head 318 ( must have sold new in Canada). I found an Iron 4 BBL intake in a junk yard, installed a Thermoquad with an adapter plate and dual exhaust with glass packs. It was the best sounding exhaust I've ever had on a car. I don't know if the Poly head had anything to do with it, but it had a nice purr at idle, never droned, and was not overly obnoxious. It did get loud over 5000 RPM with the TQ low growl and the exhaust rapping away but it never had the nasty glass pack sound