Henry is right, swap leafs.
It is really hard to tell if a frame is tweaked when there is a bunch of weight on it.
If I am looking to see if a frame is tweaked I put the front of the frame on 2 jackstands, and the rear on 1 in the exact center. That way the weight of the frame is not influencing the way it settles on the jackstands. Then put a level on it. I usually don't worry too much about anything less than 1/2" out on a truck frame when it is bare. Those f100 frames twist and rack a bunch when you are driving anyway.
If you want to see if it is diamond shaped, do the plumb bob thing as suggested ,or just measure from corner to corner.
I have yet to see one that is within 1/4"
Make it so it sits level when the suspension is loaded, and drive it.
If it is way out then it's the frame shop.
my signature lines...not really directed at anyone in particular..
BE different....ACT normal.
No one is completely useless..They can always be used as a bad example