I don't own a 51 Merc any more, so I'll just add my opinion. My preference is to always use original parts when possible. If there is a good radiator shop near to you, a couple of considerations would be to have your radiator repaired/restored, or to buy a good original and have the same done to it. A properly done old radiator, showing no damage (tubes broken etc) should be good for many years use. It will be properly sized and will look right.
Now, I am going to my shop to remove the top tank, and rod out the original radiator on my 56 t-bird. There is no radiator shop within 50-60 miles of here and the rodding process is not difficult (I've done 2 before.)
Good luck.