[Espeefan]

For projects like this, you might consider searching for a die-cast model, or a plastic static kit, of the truck you want to build. You can get some pretty good measurements that way, and scale them up, as needed. Even if the model is not 100% accurate, it should get you in the ballpark, and provide another source of dimensions to double check your drawings. Now if there ever was a Pete 320 truck model produced in die-cast metal, or as a plastic kit, that is another question.