For Atlas Classic GP7/9 locomotives, a UFIT-7535-RC1 will mount perfectly to the frame behind the flywheel within the long-hood and a UFIT-5935-RC1 will mount to the frame in front of the flywheel within the cab/short-hood. No frame/weight/shell modifications are needed! Decoder installation is simplified with a Nix Trainz Decoder Buddy and a 21-pin decoder or by using a Loksound Direct or Tsunami PNP installation.
If only installing one speaker, I recommend the long-hood UFIT-7535 for a deeper sound. Adding the UFIT-5935 provides better sound-localization, thicker mid-range and more overall possible volume.
The speakers come fully assembled and tested.