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Keep in mind that there were only 30 of these units.  There were hundreds of GP40-2's.  You only need one of these to be representative of the fleet.  Throw in an Athearn U30C or U25B and you basically have the GE fleet done.  GE units tended to stay near Virginia, so to find them on the East End was not an every day occurance.  If you are modeling a C&O line, then these units would be more common.  Remember to be a prototypical model railroad you need the feel of the line.  Too many GE units will give the line the feel of Virginia not Maryland/West Virginia.

GE units had two main problems in the Chessie era.  One, their paint was absolutely dismal.  It faded, peeled, and rusted through all the time.  Two, they are not preferred by the crews for their slippery handling and noise level.  So, with this in mind your GE units can be either dummies or powered, but most likely a dummy is an economical way to go.  GE's tend to be trailing units in a consist.  Place the GE in the middle of the pack for a prototypical look.  Newer engines tend to be on the point of a train.  Engineers like to ride in new seats and clean cabs.

Below is a pic of a GE U23B.  It is a Dean Heacock pic.  Notice that this unit is not at the front of the train, and there is an even older GP9 behind it.  This is typical of a Chessie consist.

As for what needs to be done to this kit, you will just need to weather it and paint a few things the right colors.  There is no need to do any cutting with this kit.  Atlas has done a fabulous job with it.  I highly recommend this kit in either the blue or the Chessie paint.  I really like the blue paint job when it is dirty and faded, so that is how mine looks.

Below are two a pics of my U23B, #2324.  For more information on this see either my Atlas U23B tips or 2324c&o locomoitve page.  The Atlas U23B page has a complete article on how to make this locomotive.