Installing 4 piece shower enclosure that's slightly too small
Since you don't yet have drywall up anywhere, I would fur one entire wall out 3/4". Then attach your drywall to that and you don't have to deal with the return. 

With openings on each wall, it's a decision of which wall is easiest to fur out. First, make sure that either one would work (e.g. if you fur out the right, make sure your drain still lines up). Then you have to make the door or window jamb work with the deeper wall. Even if the door wall is easier to fur out, I'd still go with the window wall, simply because a prehung door &amp; custom window would be easier than the other way around.

And completely unrelated to your question, but something I wish I knew when I put in my first tub - pack the tub full of insulation, in addition to the walls. A piping hot bathtub cools off mighty quickly.