The picture to the left is the back assembly. Line up the top edge of the two pieces. The other edges could be sanded flush easily with out changing the alignment of the hinge.
The picture on the left is an ok picture of the chassis assembly. I used the axle pegs to line up the wheel spacer on the chassis. Just make sure to remove the peg before the glue dries.
This design is noted in the plans but not shown. The design is good as it is. I know how by kid throw their toys so I wanted to add a little more support.