It is probably possible, but I'd suggest waiting until you can do the whole thing. The tub measures (typically) very close to 60" (this is the most common). Wrestling the thing in there if you can't tip it can be a problem. Measure the opening now. You may find you'd have to remove some of the drywall on the outside as well in order to slide it in or take off more than a few rows of tile.. Keeping the vapor barrier intact is also harder to do.