Athearn's GP40-2 is a great starting point. See my "GM50-one of a kind" page for more details on how to build the kit.
Below are Dean Heacock pics of #GM50.
This unit was repainted in August 1984 into a standard Chessie paint scheme
and renumbered #4164. It carried that number for a few years and
was repainted into the CSX paint scheme and renumbered again to #6063.
#6063 is still out there, I have seen pics of this unit with the paint
peeling and the old gold paint showing through. This unit has been
earmarked to be given to the Smithsonian Institution apon retirement.
Below are pics of my Athearn GM50. It has Herald King decals and lots of detail parts. No Chessie layout can be considered complete without this flagship locomotive.
Here is a starting pic.
Here is what it looks like done.