Secure a 2x4 on the back wall to support the back ledge.
Move the tub into the alcove, drain side down, and keep it in the box.
Get the tub as close as you can to the wall with the valve.
Have two 2x4's on hand.
One will be your pry bar, the other will catch the edge of the tub apron before it touches down. Lay that one on the ground now.
Next, take the crating off of the tub.
The tub will drop down out of the crate when the wire ties are undone.
Now, start dropping the tub down into the hole.
The tub will drag on the back wall, catching on the studs.
When you have the tub almost all of the way down, it will be stuck on the wall opposite the valve.
This is when you use the second 2x4 to pry the bottom of the tub up on the drain side.
You can use the second 2x4 as a pivot for the lever.
You work this combination until the tub drops into place.
You may have to sit on your butt, and push it in with your legs.
Normally a plumber can set these by themselves, once they are in the bathroom.
It helps to see someone else do this first.
After setting a few, it becomes second nature.