You can start to narrow it down by knowing that there was a total of 71,775 Chevelle's and El Camino's produced at the Fremont plant.

Oh boy, that is going to be tough information to find. After 1957, Chevrolet did a poor job of keeping track of production figures. So far the only information I can find is that 41,791 El Camino's were produced overall and out of those 5,190 were El Camino SS. I'll keep looking though.