The scatter rules in the Core Rulebook, I believe, talk about moving the model just a bit less or just a bit more than where it "lands" if it would end up on top of another model, so the same would apply, I suppose - either stop the drill short of the ground unit, or shimmy the ground unit to the side. You decide.
It's not heart surgery. Its a game. You can be off by a few centimeters. Discuss it with your opponent before the game. One thing is for sure -- this isn't 40K, so none of this "Well, if you can't place it, it is destroyed, remove it from play" bullshit.
