Have a look at Dog Bar Rd into GV. Less traffic than 49.
From NC, Broad St to E Broad to light at 49; cross 49; North Bloomfield Rd to T; left on Lake Vera Purdon Rd; follow it across South Yuba to Tyler Foote Rd. Cross Tyler Foote and take Oak Tree Rd to North San Juan.
Then it's 49 to Downieville. Or take the road to Pike and Forest City, and trails to Downieville.
If you watch the weather forecasts, you might be able to skip the tent and just use a sleeping bag.
Why rush? It makes for a more enjoyable ride when you take some extra time to explore and savor the roses.