Most of the replacement oil sending units for gauge equipped cars have a square shaped part at the top of the threaded stem or nipple that threads into the block, you cant see it well because of the bell shaped portion of the sender is above this part. As I recall they are about 1/2 inch square. On a 383/400/413/440 if you get the air cleaner out of the way and situate yourself just so you can see how they are built. A shortie wrench will usually do the trick. Mikey 62 Crown Coupe