I think what Lugnutz is saying is the fill cap is in the top tank. That would put it on the drivers side.The bottom tank is on the left in picture.
Your picture shows the radiator on its side.


"Do not walk behind me, for I may not lead. Do not walk ahead of me, for I may not follow. Do not walk beside me either. Just leave me alone, you're starting to freak me out."

1957 GMC 150