Deceiving exhaust sound

Hello there !

I have a Dart 67, originally a Slant 6. I put an absolutely stock 318 in there. I self made an exhaust system and it doen't sound good. It's a dual exhaust, each pipe ending just before the rear wheels on each side. There is no connection between the pipes. While driving, it sounds like a flat four :sad3:! but when I'm outside, with a friend driving and passing by, it makes a good sound ... Where's the mistake ?
Any help will be greatly appreciated.:-D
thank you all for reading.