Murray,
Another alternative you may consider is to go through Lake Arrowhead. You may be better served because of your low ground clearance. And you can figure you time difference later. Another advantage is the fine restaurants there at the Arrowhead plaza. There are no dirt roads to mess you up.
To get to this route from L.A. area(and I am using my memory):Take the 330 exit from the 15 freeway go a few miles to the Waterman Street exit. The signs will say Mountain. resorts. Go up the mountains till you get to the Lake Arrowhead left turn, (several miles). After you go @2.5 miles on this road you come to a stop sign. This is the highway173 route. Turn right and go @ 4-5 miles you will notice the houses will not be around then a forest rangers station on the right, then a shooting range on the left. 1 mile past the shooting range is a cement bridge this is the McKinley creek bridge you may recognize from your previous trips. Park near here and follow the Bradford ridge trail which will be on the right side as you reach the bridge.
Good Luck and let us know how your trip went and the time difference.
Glad to help!