I've thought that if I ever get a yard barn, I'll paint the mailpouch ad on one side, See Rock City, The Lost Sea and Ruby Falls on the others....
You could get a good picture of a mailpouch barn in your computer.
Use a picture editing program to put a small target X in each corner.
Next, rent or borrow a projector and project the picture onto the barn at night and paint the outlines.
If it takes multiple nights, use the target X's to line up the picture each night.
Once you get the outlines, you can finish it in the daylight.
I've seen professional mural painters use this technique many times. It works well.