Your best solution is to relocate the position of your new box. Go around the wood, don't force the situation.
In situations like this, I have taken to using Forstner bits instead of spade bits. The tip is not as deep so it prevents the tip from hitting other things in the wood. The bit is flat bottomed, so it is more precise. The more I use these bits, the more often I find uses for them ... Sure, they are primarily for precision woodworking, no hogging out wood, but if the tool works, use it!