The Siskiyou's can be really if'y in winter and I would really suggest that you take chains with you. I heard that Les Schwab would sell you chains and if not used, you could return them. I'd also have a weather radio as I've know them to close I 5 south of the Oregon border when it starts to snow. If that happens, 7 Feathers RV would be an excellent point to pull in and wait it out. Since it's winter you shouldn't need reservations
350 miles a day is really ambition in winter when it starts to get dark by 4. We like to be in camp, set up and with a cool beverage before that, so we usually try for 250 miles a day, but that is a personal decision, and I can sure understand getting out of the cold
Also, I would think about taking the Needles I 40 also. Probably the most boring road to drive, but boring isn't bad after a long trip.
