Which engine would be the most compatible for an engine swap in a 2010 Galant?

Give up before you even start butchering the car and go buy what you want. Professionals would never try to do what you're planning because they know what problems they would run into. Converting from a 4 cylinder to a 6 cylinder means replacing the front coil springs to handle the extra weight, replacing the entire braking system to handle more weight and to keep the carefully designed front-to-rear braking balance. Larger radiator, different engine wiring harness and Engine Computer.

With the way manufacturers have tied every conceivable part of the car to some type of computer, you can expect the Body Computer and instrument cluster to be different. If you have a Trip or Traveler Computer that displays things like miles per gallon, it bases its calculations on fuel injector pulse width which will be different for different engines.

Even if it were practical to pursue such a project, you would want to know where there are two or three cars in the salvage yard like what you want to build because you'll be making numerous trips to scavenge parts and take measurements. What you ARE going to end up with is a pile that doesn't run and is butchered beyond repair so you won't even be able to sell it except to a junk yard. I've done modifications like this on early '70s Chrysler products where they were well known for parts interchangeability between years and models. Those days are long gone.

If you want to convert to manual, the engine need not be replaced, but I fuly agree with caradiodoc and would not recommend such midifications, unlesss you can get a complete donor vehicle and move everything over.