Been there....done that. In N scale the shells from GP7's,9's 18's and 20's will pretty much interchange. On some combinations, you may have to remove a tab, or cut a new slot, but they fit on the chassis, and fit under the cab slots. Atlas shells are one piece (at least all mine were) so you have to cut the shell under the cab. The GP20 does not have the correct fuel tank for the other GP's due to lack of airtanks....My solution for that was to use a TT long hood. The fuel tank is still a little large, but most people don't notice. The GP20 nose also has a bit of slope to it where the other GP's had flat profiles....but a lot of chop-nose GP's were done by the railroads, not the factory, so there are a lot of variations out there.