Installing an X pipe will smooth it out a lot. Its not a cheap solution but it does work well and will add performance. I just went through some of this with my 340. I run an MSD ignition and after changing the bushings to limit the advance my motor sounded a little tamer than before. My initial timing is now advanced to 20* with total being right at 38*. Your distributor can be modified to do what you want, it's just so much easier with the MSD because the adjustments were built in when it was designed.