Keep in mind that moving closer isn't really enough, depending on how hard you actually through. For instance, you can simulate the reaction time difference if you throw 40 by throwing from 30' (please use an L-screen, though ), but the ball is still only going to be going 40, which will make the split second adjustments that result in squaring the ball up easier than they will be vs a real 80.
The best thing is to get them more reps vs a real 80. Easier said than done, of course.