She looks like she is squared off already...this happens when a guppy is extremely close to dropping and looks like she has "corners" on her belly...I'd say she is extremely close to dropping..especially if she is being solitary and hiding away from the other fish..when my late guppy looked like this, she had her babies withing hours.
Another suggestion, I would definately take her out of the breeder box...if you really want to save the babies, or if she is getting stressed out by the other fish trying to chase her for a fresh snack (sad, I know) then wait until you see her actually dropping the babies to put her in...it will just stress her out.
P.S. are all of your water stats okay? If you save the babies, do you have a home for them so your fish population doesn't go out of control? Because let me warn you, these things breed like RABBITS!
Good luck!
Meg