South in L.A., the County Museum swims upstream. Instead of letting private enterprise splatter more young artists than quality can embrace all over the Thomas Bros. street atlas, it selects L.A. 6/SUMMER ’74 and enshrines them in successive shows in a little house within the contemporary art gallery. Everybody knows LACMA gets but a trace of a pittance of the County’s three billion dollar budget, and that the modern art cadre within it receives, since “Art and Technology,” less than a fair share to spend. What Maurice Tuchman and Jane Livingston do show seems to tread a ticklish thin line