That is definitely weird, to build something from human skulls, but I think bones are not bodies, so it is much easier to move bones, and 6 millions by 2 years is someting near 10000 skeletons a day. One wagon easily could move, say, 200 skeletons, so you need only 50 wagons a day. It is definitely possible. But it is not very clear who and how dig them out. That will be narrow place in that project.

That is definitely weird, to build something from human skulls, but I think bones are not bodies, so it is much easier to move bones, and 6 millions by 2 years is someting near 10000 bones a day. One wagon easily could move, say, 200 bones, so you need only 50 wagons a day. It is definitely possible. But it is not very clear who and how dig them out. That will be narrow place in that project.