I don't think Shasta is considered part of the valley -- there are 20-30 miles of hills/mountains separating it from the northern end of the central valley (Redding).

That far north is definitely outside the central valley. Once you travel north out of Redding, you're climbing up into the mountains, and it's 60 miles of that before you get to Shasta. Lassen is better candidate, since it actually erupted recently (1921) and is only ~40 miles east of Redding... but it's also part of the Cascade range like Shasta, which sn't really IN the valley.