sure would love to get my hands on a z car. had a 76 280Z back in the day(83-85). that inline 6 would shit and get!I'm using the walker quiet flow that @Irun referred to. It's as much sound as I want.
I blame the 4.0s exhaust manifold design; it just doesn't sound pretty. I know inline 6s are capable of sounding good because I had a Datsun 280Z with a 40 series flowmaster and no cat that purred but I also had a long tube 6-1 header and an ITB carb setup on it.