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I was having trouble writing and adding a picture as well. Anyway the above picture shows the contrast in size and spline count for the T-bird Supercoupe and the Cobra.
I can't imagine finding an axle 5" shorter. The  Factory Five guys use these axles in two different lengths.
One for standard hub and a shorter axle for the pin drive hubs. These guys all have to have them cut and resplines. So if there was a shorter axle someone no doubt they would have written about it. Everyone prefers to save money. The good news is the cost of cutting and resplining an axle is the same whether you are cutting 2" off or 5".

Well, I have heard the number 400-500 HP bantered around which isn't shabby. My Tremac T-45 is only rated at 330ft/lbs torgue. Following the posts on Factory Five forums there are a lot of high horsepower
Cobras running this IRS. Now I do realize that if the tires simply spin and don't hook up then it doesn't matter if its 200 or 2,000 HP. The IRS is not being stressed.
One FF guy Sergio has built a Cobra using a supercharged motor from a 500GT Cobra mustang. He used specially built (expensive!) axles. However what happens is you move the strength upfrom one weakest point to the next weakest point. So I think the problem then becomes the stub axles. I understand that going from 28 spline to the 31 spline will help here. Anyone able to elaborate on this?
The guys that race Fords tell me that the 8.8 differential is every bit as strong as the Ford 9" which was the differential of choice for year. Keep in mind though that they are using the 8.8" in a live axle not the same parts in an IRS.

I have owned a 7.5" Ford IRS from a T-bird, a few 8.8" Ford IRS from the Super Coupe and the Lincoln VIII and from the Cobra. I never measured the axles but have had them all together being cleaned of grease, having universal joints removed etc.. I have never noticed any difference in length. Width yes. Spline count yes but not length. The 7.5" use a 1" axle 28 splines each end. The 8.8 use a 1.2" axle with 28  splines in each end. The Cobra has a 1.5" axle. From there I am a bit confused. On the wheel side the spline count is 28 but on some the inner is 28 and later years it is 31. I used the 1.5" axles on my build. As I recall I did not have to deal with the spline count in the differential. I had the cast iron 8.8 and the Lincoln aluminim 8.8. I simply used the tri-lobar (is that what its called) from the Cobra in the Universal joint for the T-bird. No problem. I needed to shorten my axles 2". It cost me $350 plus freight to have then respline then rehardened and annealed. I know nothing about the Explorer IRS except hat several kit car guys tried them and gave up. Fell into the too hard basket and they went back to the T-bird. Could be shorter axles but I doubt it. The Explorer isn't exactly a skinny car. Not going to be skinnier than a Mustang.