Decoder & GP-7 trouble

[MaineCentral]

 I would like to ask for suggestions on installing a DCC decoder into an Atlas Classic series locomotive.   
     Here's what I'm working with: Recently purchased GP-7 Item #8636.  For this model a local hobby store recommended a Digitrak DH163IP decoder.
     My problem is that during re-assembly the body shell seems to be "held up" by the newly installed decoder.
     It appears to be caused by the wiring of the front head lamp.   Due to the new decoder it now rests in a higher position causing the shells not to rest on the frame. 
     My question... What am I overlooking?  Do I have the recommended decoder? 
     I'm new to the hobby and not very confident with my knowledge.
     I'd appreciate any help you can offer. 

[RAM]

I think on an atlas if you use an atlas decoder you just remove the light bar and replace it with the decoder. 

[jsmvmd]

Check out Lionel Strang's book on "DCC Made Easy."  One chapter is devoted to a decoder installation in an Atlas diesel.

Best, Jack

[Hunt]

To use the Digitrax DH163IP
Workaround --- Turn the  locomotive PCB  end to end.
For more details see info in TCS website   http://www.tcsdcc.com/decoderpics/Atlas%20GP7.html

[Jake]

Check out Lionel Strang's book on "DCC Made Easy."  One chapter is devoted to a decoder installation in an Atlas diesel.

Best, Jack

Nope, its actually an Athearn diesel.