I have a large number of Atlas cars on the railroad that in my experience would not reliably couple and uncouple (hands off) during operating sessions. To address the issue I decided to standardize draft gear using Kadee 800 series couplers. (newer Kadee 700 series couplers look better, but in testing I've found the old design 800's couple and uncouple more reliably - especially on slight curves). With many cars to convert and absent a bolt on/drop in replacement from Kadee (I've called and urged them to produce one) I developed some shortcuts for mounting the Kadee pocket.
As others have posted I do not mount Kadee pockets using the existing AtlasO mounting holes as that results in the Kadee pocket extending beyond the end of the car in a non- prototypical fashion. To minimize drilling and tapping operations I mount the Kadee pocket using only the center screw. A single 2-56 (or 2mm) machine screw is more than strong enough to resist the longitudinal (shear) forces on O scale draft gear. Kadee (and Atlas) use the second mounting screw to keep the pocket from rotating off center line due to lateral forces on the draft gear. By mounting the Kadee pocket's upper lip firmly against the end sill along with a drop of ACC super glue I've found that the that the second screw is not required. To locate and drill the hole for a new Kadee mounting screw I made a simple guide fixture using the base of an AtlasO pocket (with the sides cut away). While drilling the hole the fixture held in place using the original AtlasO mounting screws , and then removed when tapping fthe new hole. I've done over 30 AtlasO hoppers with this approach and found it to be fast and effective.
Note the small locating hole in the fixture 
drilling the pilot hole
tapping 
a drop of ACC 
