The mains are machined and ground, between centers. Meaning the crank is supported on both ends, only. They are done that way to keep everything on one plane.
If your crank will sags, you need to get rid of it, as it's not very strong and won't survive.
W/o a lathe available, supporting it on #1 and #5 with 2,3& 4 bearing shells removed, would be acceptable and akin to supporting it in V blocks.