Are What RPMS should a /6 be at while driving 55?

I thought the 318 in my Duster was spinning awfully fast at 75 MPH but when I hooked up a tach it was only spinning around 2600 RPM, and that's with a 2.76:1 rear end. Funny too, my redline was around 3500 RPM (hopefully a rebuild/hot-rod tricks will cure that).