turn it so the tub faces the bathroom door and the bottom of the tub faces the back OSB wall, as much as possible. Since your drywall isn't up yet, you have a couple inches extra space on each end, to turn the corners in, when it is turned on its side. The length is much less on this 18" x 60" diagonal than if you used the 30" x 60" diagonal.
An 18" height need 63". Might fit. Calculate in advance if you wish by measuring the diagonal and the max distance available. Or we can calculate by squaring the tub height and the tub length, summing these two numbers, and square-rooting.
if it works, it puts the tub in a straight line, but turned on its side. Then from there you are home free. Rotate it on its length axis into place. With a helper. Pad the floor. Get a strong patient helper. The tub (it's acrylic, right?) weighs less than 100 pounds.
i think you may have to remove more drywall on at least one side, and i hope your plumbing isn't taking too much space between the studs. Last step if necessary, using this same side-turn method, is to cut two 1/4" grooves in one stud (or remove a part of it and replace it right after), to let the two corners of the tub slide through.
hope this helps. and no i have never done this.