The quickest way to improve a football team is to sign veterans in the trenches (OL and DL) and draft rookies in the backfield (RB and LB). Linemen need time to develop experience/technique, whereas backers can make an immediate impact on athleticism alone.
So if the Niners want a quick-fix, then I'd say:
(1) With their first-rounders, draft two athletic backers: RB and LB.
(2) In free agency, spend money on a veteran *starting-caliber* OG/OT.
(3) Fill out the rest of the offseason with depth at DL and DB.