Your original 635pm departure has only 2 seats in Y. It's going to be a full bird.
IAD is definitely your best bet.

** AMADEUS AVAILABILITY - AD ** SFO SAN FRANCISCO.USCA 0 FR 30DEC 600P
1 UA 951 F8 C8 Y8 B8 M8 E8 U3 /IAD SFO 3 500P 801P E0.752 8 6:01
H3 Q3 V2 W0 S0 T0 K3 L0 G0 P6 A6 D5 Z5
2 UA 189 F4 Y9 B9 M9 E9 U9 H9 /IAD SFO 3 610P 902P E0.752 N 5:52
Q9 V9 W9 S9 T9 K9 L7 G0 P4 A4
3 UA 187 F6 Y9 B9 M9 E9 U9 H9 /IAD SFO 3 740P 1030P E0.752 7 5:50
Q9 V9 W9 S9 T9 K9 L9 G9 P6 A6
4 UA 205 F9 Y9 B9 M9 E9 U9 H4 /IAD SFO 3 945P 035A+1E0.752 8 5:50