I'm also a lefty but retrained on pistols to shoot right handed left eye dominant.
The AR, has been a whole different animal for me. I gave it a try doing it left handed just to see what it was like but got tired of brass raining down on me. So now I am in the process of TRYING to learn how to do it right-handed and right eye dominant. My problem is... I am SO left eye dominant that I almost feel vertigo trying to get my right eye to take control and focus. It can be done, it's just challenging.
I am determined to live as a righty in the gun world though.
Definitely get the shell deflector, if possible, if you are going to shoot left handed with a right handed rifle.