69 Valiant Engine Swap?????

I just got a 69 Plymouth Valiant, it currently has a straight six and an auto trans. This is my first build and I would like to do all of the work on it myself. I would like to swap out the engine with a 340, 360 or 383 but don't know witch one would be the right one for me to put in the car. I really would like the 383 big b and I know a guy that has one for sale, he told me it would fit if I went to manual steering but I don't want to lose power steering. I am looking at putting in a manual trans but didn't know if that would be a difficult task or not.
I could really use some solid advice on this issue, Thanks..