I think they are 28. I'm not for sure but I think the 7.5 and 8.8 axles are interchangable after about '87. There was another axle that was on the early 80's 4 speed & autos that was tiny, and I don't think anything would interchange on those with anything else.
My observations are that at least 79 to 89, 7.5 and 8.8 axles are 28 spline and are interchangeable.
I have used 79, 80, and, or 82 axles in 79, 82, 80, and 89 7.5 in housings, and in my 89 8.8 housing.
Yes, the early 79 era Mustangs with four cylinders had the Pinto 6 in-ish axle. I did not try the interchange before I scrapped it.
Note that there are some mid 80's T-birds with identical housings, but longer axles, and a wider brake drum. Those axles stick out too far on a Mustang, or you would at least have to use the offset drums. Please do not ask why I know this....