Barber, Jackson, Calvin, and Rowdy Roddy are the easy picks but flex is the obvious one giving you headaches.
I think you have a great TE that Cutler will rely heavily on this year but not having to start a TE, I would have to lean towards another WR.
Being a fellow Niner fan, I admittedly stay away from Niners on my FF team because it clouds my judgement and this week's game can go either way. I can see Gore controlling the clock or I can see Arizona lighting it up and forcing the Niners to throw to stay into it. I can see Morgan having a great game or being irrelevant.
For that reason I would go with the likely more consistent option in Santana Moss.
Good luck and GO NINERS!