Do yourself a favor and put a three piece tub in. They are built as well as most one piece units and you can shim them to fit much easier.   we installed one in the house years ago. The walls were about as yours,not to straight. floor was a bigger problem as it ran backwards to the drain. Used a skil saw to channel the tub front so it would go down in the floor level... To do this you need to mark the line then use a nice straight 2x4. level the 2x4 so that when you cut it will cut a level slot no mater the floor.  we also put a cap on it and it makes a nice unit.

  Course after all these years the boss thinks blue is out of style.