Histories come in all shapes and sizes and can be chronicled in any number of fashions. Family histories, each unhappy in their own way, may reveal personal pains that turn out to be strikingly universal. A country’s history can be told in terms of its politics, its geographic landmarks, its immigrants. And a people’s history can be reflected in its folk music. Any one of these might make for an engrossing night of theater. But when attempting to combine all three, finding the right balance and weaving a cohesive tale become a tall order. Such is the case with the New Light Theater Project’s production of The Greatest Hits Down Route 66, which finds itself short on songs, long on family dysfunction, and scattered on Americana.