I'm in the exact same weather pattern, up here in West Sacramento. I know Santa Maria quite well.
8 weeks is the time I put my chicks outside full time. These overnight temps in the low 40s have given me pause, too, but mine won't be 8 wks old until Memorial Day. From their behavior - this batch is precocious! - I would have put them out this past weekend except for the rain storm coming through tonight and the lower temps again. My extension cord won't reach to where I'm going to house them segregated from the grown up chickens for a while.
MOST chicks are fully feathered by 8 weeks. The only reason I hesitated in putting this batch (a whopping 4 chicks) out is the fear of temperature shock - they can take the lower temps once they acclimate, and I hadn't been turning the heat lamp off at night lately. They complained when I tried, when those recent night temps dropped (even in the bathroom). Sooooo...... no heat lamp this week inside the house.
If you can stand it another week, put them out at the end of this week, when this wonky weather stabilizes a little bit (or so say the weather cast folks).