I believe the factory originals were done in Organisol Black.
Due to the high VOC content of that paint, that stuff is no longer available from the paint manufacturers (you can thank your friendly neighborhood environmentalist for that one) and the cross codes to the newer stuff don't come out exactly the same or as nice from what I can see.
IF its not an original restoration, I'd go with a flat or semi-satin black.
You can still find Organisol Black from some of the speciality dealers, but be prepared to open the checkbook. Fortunately, that stuff was lacquer-based and if stored properly, will last a verrry long time on the shelf. The Phord mustang boys are in the same boat as a variant of it was used on alot of the early mustangs too.