You could use an old boat building trick, a water level. It's a "bucket" or can of some kind with a long clear tube out of the bottom. Place the end of the tube ABOVE the level of the bucket and fill the bucket with water. Now position the bucket so that the water in it is at the level you want the line to be. Take the open end of the long tube any where along the truck and the water level there will be at the same level as in the bucket.
This is how you "scirbe the water line" when you build a boat upside down.(Wooden Boats mostly) There are NO STRAIGHT LINES on a boat and this works great. Sometimes a little food coloring in the water helps, too.
