Have you checked the road conditions on Hwy 299? I just rode a tour that was on 299, but the east side of Redding, to just east of Burney. It sucked. Big time. Much of it had zero shoulder, fast traffic, and lots of logging trucks. Had several close calls. I won't ever ride it again unless that changes. My guess is you'll get a lot of that in the area you're considering, but the other highways will probably have light traffic. That area will also be hot this time of year, until you get near the coast. Speaking of the coast, have you considered riding that? It's beautiful, cool, and after labor day the traffic will lighten up considerably.