This has to be the best case scenario for the 49ers. I doubt Williams will fall out of the top 10 with the combine he had, but if he does this is the pick. Williams would start right away at right tackle and maybe compete for the left tackle position pushing Joe Staley to the right side.
If Williams, Davis, Okung, or Bulaga are gone with this pick the 49ers should draft a tackle later on. At this spot a tackle should at least have the potential to start at left tackle.
If one of the top four tackles isn't available with this pick the 49ers should try and trade back in the first round and obtain an additional pick. Cornerback is another option, if Haden is around, but I think Joe Haden is overhyped.
This draft has depth at cornerback, which the 49ers must add quality younger players.