Not sure what engine angle has to do with it unless there is a common firing order the 60 degree v6's all have. I will say from messing with many a Buick the 6 cylinder has some strange noise attributes or harmonics that are hard to tame. Theres a reason whe they are called, "A Buzzing half dozen". Try calling Borla, does there system stay small or does it expand as it goes out? If I were building a NA car I would be running Borla or Kooks headers, an X pipe and 2.5" exhaust out the back, this is just based on things I have seen and experimented with over the years and may not be the best.
Example a buddy with a 89 5.0 and I had similar builds, the only real difference was he stayed 2.25" going into his mufflers where I went 2.5", my car was 1 mph faster, we swapped exhaust and his car picked up the mph and he went 12's for the first time, not talking 800 hp cars here, talking 275hp. grand national burnout - YouTube