There is a big difference in running a 10 once and having it "hold up". One good hook on a 7.5 has been known to send the spider gears through the back cover, why roll the dice?
You can buy a good selection of gears and parts for the 7.5 axle. I have two that I have installed FRPP traction lok units in, includng one that I put FRPP 4.10 gears in, with the traction lock. I have used the Reider Racing stackable carrier shim kits with good results.
A lot of the 7.5 stuff is more widely used in the Ranger world, but it all fits Mustangs.
I did it because I started in the mid 80's when that's all there was. Then, because I had 7.5 stuff, I kept them. Early opinion was that you could get into the 12's on them, after which you needed to upgrade, and at the time, that meant a 9 in.
I have a bunch of runs and sprayed launches on my 7.5 with the 4.10s, and 28 spline axles.
However, I do have a couple 8.8's, and had one in the car for 1/8 mile use.
If I was starting today, I probably would try to spend the money once, on 8.8 stuff.