Pypes A body exhaust or Summit Which one should I choose?

I used Jegs version of this kit, not sure if it is Pypes or Heartland or whom-ever, in 2 1/2". Kit was for a 67-up car and I installed it on my '65 Valiant w/318 and Spitfire headers. Total cut and paste deal front to rear. I used a home made H-pipe and ran the exhaust to the back bumper with stock style turn-downs. I have a hell of a resonance from 45 MPH to 60 MPH, much worse with the windows closed. This is with the provided generic "Turbo" mufflers. That is the only complaint I really have, other then that the system is very mellow sounding.
Bob