As Pete stated your radiator is short for a full size car. It may have been sourced out of a mid size or a compact car which has a smaller radiator due to restricted space (why it is so hard to cool a BBC Camaro). I used a radiator out of a BBC light truck which is bigger than an Impala radiator to cool my maximum sized (for a standard deck height) BBC.
If you look at a Semi truck you see the size radiator that is required for a 350 horse engine (that is all the horsepower an in line six diesel will have on average, though a semi has many other engine options up to a V12 available to it if you don't mind buying a lot more fuel to get you to the same spot). That is why a Semi can idle all night long and not over heat. With radiators bigger is better.