Well, in that case, I would suggest making the back end two studs wider and making the back end longer. That way, it is more in proportion to the front. Notice the size of the back end in the link you had posted, and the proportions of your MOC. As for the pic you posted, that seems completely different.
Believe me, I'm great at drawing and evaluating trucks, cars, and MOCs, but when it actually comes to making them, it takesme too long to do them. My Halo mocs will be done though...
Hope that helps,
-VbBn