The fuel line was replaced or worked on at some point in time. There are two clamps on it and the pickup fuel line doesn't need two clamps and may not have had two originally. If they were factory, the clamps would not have been installed facing the same direction.
I can't tell from the picture if all that crud is just dirt and caked slime or if it is corrosion. Regardless, tanks usually develop pin hole leaks on the bottom side.
As Dennis mentioned, those are not stress cracks. .
As I see it the cracks indicate that the forces were not towards the transom but rather from the inside aft. The largest forces typically are against it from outside as the motor created thrust. Since your crack spread in the opposite direction, it seems that the cracks are the result of one or more factors: The F150 is heavier that the original motor and the extra weight would put extra load on the transom but unless weak, it normally shouldn't crack. Another possibility is that the motor had one or move collision with something not so soft. That could cause large forces on the transom.
Did you hire the surveyor and if so did you get a written report? I would like more detail on the hull inspection, specifically moisture readings on different areas of the hull. When was the last time that the boat was in the water prior to the survey? Note that transom with a hollow core will measure little to no moisture so comparisons with other areas of the hull are important. If the core were hollow, I would expect the hammer test to detect the condition by comparing the hammer sounds as the hammer is moved around the transom. How did the skeg and prop look?
As some may have noticed, I am a skeptical by nature and always lean towards worst case scenarios