I realize this is an old post, but I'm also on the search for the correct color for my early 1966 Mustang 289 engine. I bought Dupli-Color 1606 and my engine is not that dark blue. I'm having it rebuilt and know for certain that it hasn't been worked on at this level since the factory, so all indicators show a much lighter blue than 1606 (Ford Dark Blue). I'm going to buy a can of 1621, as it looks like the closest to what I have now.
The attached photo shows the cap of 1606 next to three of my engine parts, and the lower right is 1606 painted onto a toploader. It's clearly too dark.