These boats are NOT square, the level Pic is included to let anyone in the future know there is not a great way to "prove" accuracy. You will be eyeballing lots of things. The stringers at the bow and stern are level and that is what I am going off of. Is that the correct way? I don't know.
The other pics are of the fuel tank mounting apparatus. Most use pvc tubes cut in half and that is a great. Probably the way to do it on most boats, but my boat's keel is off 1 1/2 inches from bow to stern so I opted for piers to mount the fuel tank on as depicted 8n the pics. Is it right? Sure, or it's not,. I will find out hopefully over the next 20 years.