Join Date: Jan 2018
Location: Coopersville, MI
I got rid of my 66 almost 24 years ago, but my memory says the square one you have looks right. If you took the one you have off it should have a part number on the plastic someplace. If someone has an old parts manual that covers your year they could cross reference that part number and verify it is correct.
Looking at the parts dealers it looks like the square one is correct also.