AtlCSIP

You are going to have to take a checkride. You don't need an endorsement or a written exam. The checkride will be an ATP checkride. If you want to get a private or commercial, you will have to have an endorsement from a CFI and pass the appropriate written, as well as pass the appropriate checkride. Getting the ATP single engine, in your case, will actually be easier. You can take it in any single engine aircraft that is instrument equipped and certified. Make sure you practice everything in the PTS. You will do slow flight, stalls, steep turns, unusual attitude recovery, 3 or 4 instrument approaches (at least one coupled, if the aircraft is so equipped), holding procedures, circling approaches and landings. If you are near Atlanta, I can help you. If not, make sure you find an instructor with a lot of instrument time.