Well a couple of things come to mind.
One is that it looks to be wired correctly at the starter. But two, it looks like you're just a start or three away from a full run-on of the starter!
More on that in a minute...
Can you post up a pic of your starter relay on the fender? Just want to see what's up there as well.
Regarding your starter, I would disconnect the battery, yank that main starter cable from it's current position and re-orient it, ore replace it's end with a 90° type.
The reason being, even if it might have more clearance than we can see in the pic, it looks like it's literally touching the starter motor's stud. I know it can't be, or it would be spinning the moment you connect the battery. It's just too hard to tell from this angle, but I would change it anyway.
Looks like you have plenty of cable to at the very least loosen the nut and rotate the cable upwards away from the starter motor.
I can see that you're trying to keep it away from the heat of the headers, and you don't want it actually rubbing the engine of course, but there is I'm sure a happy middle ground in there.
On a side note, I like to use the proper sized shrink tubing on the semi-exposed ends of these particular style cables. For your garden variety store bought cable, they're actually very good at what they do. But sealing the exposed strands where the outer jacket almost meets the edge of the connector is just going to give it a longer life before it gives you any grief.
I do it at the starter relay ends too, but especially down at the starter. That's an unfriendly environment even on a show car. Drive it and that poor cable sees a lot of grief.
If you ever have it off again, slide some shrink tubing over the end to seal that joint.
And if you do have it off for any reason, remove that loose jumper wire too. No reason to keep it in harm's way. And to just give the whole thing that cleaner look I'm sure you'd like with the Mustang.
But no matter what you do, rotate that cable up and more out of the way.
Sorry I'm not sure why yours is experiencing run-on. Maybe a pic of the relay will highlight something.
Paul