USAF maintenance training for my AFSC (job code) was on the B-52. It was stated that normal wing flex at the tips on the BUFF was +/- 7 feet. So that's a total flex of 14'
The B-52 is a high wing aircraft with an anhedral There are outrigger gear on the wingtips of the BUFF to prevent wingtip strikes. When they were heavily laden both outriggers could be in ground contact but normally they would not be touching the ground. Also there was some differences on the tip tanks that were mounted on the different models that may have had a definite effect on wing flex. One of the other interesting things about the BUFF is that during climbout you would actually have a negative deck angle. It was pretty wild to see them climb out with the nose down!
The 747 is a low wing with a dihedral Two very different design concepts, between the two different aircraft. I would have to say that the wing flex would be very different as well.
Not that I can get all that good of a look at but by observing 74's during takeoff but the wing flex on a 742 does not even come close to that. Have no idea what the normal flex would be on a 74.