Yes, box that in, its going to have a lot of stress! Do you have a picture of how the motor bolt is hooked to your mount? I can't help but think that you need a more robust mount. This is sleek but I'm curious of its functionality. All motor/street vibrations will go through that small amount of steel.
You will also probably quickly find you need that rear stabilizer. The rubber-mounted front likes to wiggle, and with nothing holding the back in place it can feel funny through turns.
Anyhow, good job! Keep us posted on how it holds up.