Guys like Mankins will be RFAs, so the Niners would need to give up high draft picks for them. That's probably not an option.
Fortunately though, at pick 13 one of the better OTs should be available (Williams/Davis/Bulaga).
But I'm thinking that pick 17 might be too high for an OG. Possibly trade down to the 20s for Iupati and get an extra 3rd-rounder? If they stay at 17, there are quite a few solid d-linemen in that area (Williams/Cody/Jones/Odrick), although that's not a pressing need.