Just thought I'd chime in with another opinion.
Since a Mustang II was the first ever car that I bought at 16, almost 20 years ago, I've been smitten with the Cobra II. So much so, that a Cobra was my second car, and third car. The fourth was a brand new 89 'stang, but anyway...
I have had all three engine sizes, but only V6 or V8 in a Cobra. There is a Cobra II here in my town, or was, that has a factory 2.3. It is the only time I've ever seen that, and the first time I saw it I was shocked that this car would come with all the bells and whistles only to have a 4 banger under the hood. Now though, as these cars become scarce, that novelty becomes a real rarity.
I've combed many a junkyard and car lot, and never passed one of these cars without stopping to take a closer look. If I had to come up with numbers based on my experience (local only), I'd say that the non-Cobras were about 60% 4's, 25% 6's, and 15% 8's.
The Cobra II's however, seem to be like the "super sport" version of the Mustang II. Given that line of thinking, one would think that the majority of people would have wanted the 8's, and I would say that the production numbers would reflect that, and be about opposite of the ones listed above.
BTW, after not having owned a II for over a dozen years (with much regret), I recently found out how nicely a little 77 Cobra II fits in my garage