I defer to the ultimate authority on the subject. I just checked Joe Sutter's book and he states that the concrete blocks were indeed to stop the aircraft tipping.
I suppose the same effect could have been achieved with a heavy pallet in the forward hold but the problem with ballast is that it can be forgotten! Replacing a concrete block with an engine can be achieved with the same equipment at the one site so it all makes perfect sense.
ummmm. well, or just simply hang blocks on the engine mounts. (like they already have figured out?)
Looking at other posts, very interesting, especially how aircraft would fly with concrete blocks hanging form the engine mounts, instead of engines?
What's the technical purpose of hanging weights from the engine mounts?
Aside from all of the tipping over stuff...the wing is designed structurally to handle the load of the engine. This is achieved with the camber and other structural elements in place. Without that load, the wing construction can actually place undue stress on the structural elements. It is called Stage stressing. You stage stress the wing during construction, and pin certain elements in stages as the wing construction progresses. This prevents overstress on the elements, skin and fasteners. With the engine load in place, all elements can be securely fastened (especially the final panels. )
Lock off the elements before the load, and there is a primary stress built into the structure (that it was not designed for)
Think about a wooden truss. Assemble the wood framing, nail the plywood to the ides, then add a point load. The skin is now stressed with the load, and the framing stress with an added point load. The initial construction would be constructed, pre load and deflection with the associated load, thus inducing stress into the assembly, increasing stress loading in operations.
As an example, using wood framing that most can relate to. You have a beam on the first level, that has a column in the 1.3 span that carries 3 other floors. You set that beam, with its camber to take the load, and nail off the plywood. Then you start adding the load from the other floors under construction to that point load on the beam. The beam begins to deflect, that plywood that was fixed at the start, pre-load, with the fasteners that held the plywood down, starts to get stressed at the beam deflects with the load.
Now, if you set that beam, and hung a weight from that point, and then fastened the plywood, the skin and fasteners would not be stressed when the rest of the structure was built....you gradually reduce the weight added as the structure is built, thus mitigating the stress in the skin and connections to self weight conditions.