Boy this is some good stuff! Very few Zephyr's were made, less than 50 I think. From the discourse here I am led to believe: The "Z" was a 'new body design' incorporating not only the new Ford 2.6 engine but a new front clip design, influenced by the Greyhound, with a flatter hood/fender front section? THIS would have then been the 'new' Ace, as opposed to say the Le Mans winning 1959 Ace body style. A subtle differerence, but significant to those of us who love such details.
Pictures of the curve of the hood/front clip make it extremely difficult to compare such a subtle change. Angle of camera, lighting, etc. can easily deceive in that regard.
I understand the FIRST 'Cobra' was in fact a converted Zephyr, not an older Ace. Kris' comment the older hood would fit "reasonably well" is interesting. If the shape was indeed changed from rounder to more flatter (OK, theres a mouthful) it may well fit 'reasonably well'.