Most Mustangs aren't really all that low. In stock form they look like 4wd's next to something like a lowered Civic. If you look at Brian Conway's pictures above you can see the little slot for a "Ford" tow hook just behind where the end of the radiator support attaches to the frame rail. Speaking of the radiator support, as far as loading/towing is concerned, it's probably the worst and most delicate piece of structure you could hook to. IE, don't.
In my opinion, the big hooks are only for use around a live axle. Ford hooks in the proper frame slots are always my preference. The towing supply people don't call them "Ford" hooks for nothing. AKA "R" hooks.
In this picture the Ford hook is leftmost. Less a pain is the GM (T) hook in the center which is a good substitute. Most tow guys are lazy and prefer to just use the quickie rightmost J hook.
Trailer stuff can be different but mostly these hooks on tow trucks are attached to a fiber strap so if anything should touch the spoiler at all it would be that. Touching the plastic lower spoiler of a '69 should concern no one. Now if it starts having pressure applied to it, that's another story. Simple tow guy solution for low riders is a few select pieces of 2x6" boards. Those and some care will get about anything loaded, short of an airbagged car with a major air pressure failure. Rockers literally resting on the pavement.
Used to be tow drivers would just sling the big hooks under and hook to whatever. After a few thousand bent GM rear suspension and a few very expensive Euro cars with bent alloy suspension parts, most of them have gotten the message. But be aware, there are folks out there who literally started yesterday, got no training at all, and barely know how to drive a truck.
Funny part is, it't not so much you have to watch getting into the spoiler at the beginning, it's when you're all the way up. Check the angle in the picture. All I had to do was not bring it so far forward and it wouldn't be an issue. And yes that's me and mine (using the properly secured Ford hooks!). Jeez a few years ago now, I really need to get on the car. It has doors now!