The Shelby/Arning drop is one of the best things you can do to any Mustang (or Cougar). Even back when they had bias ply tires and suspension settings were a bit different, these cars were designed to understeer by rolling over onto the sidewalls in a hard corner.
With the top arm at a steep angle sloping down toward the wheel, as the suspension compresses it forces the top out, and the bottom of the tire in.
The engineers who designed this system weren't actually idiots - they were under pressure from safety experts and insurance companies. Even today, having a mild amount of understeer is desirable for production cars, because 'sliding into a ditch' when you go too fast into a turn is not as much fun as hanging the tail out. It also allows for more predictable handling.
Doing the Shelby mod doesn't eliminate understeer, but it reduces body roll and helps the tires do their job, staying flatter on the road and restoring much more neutral handling characteristics. It will reduce the height in the front by 5/8".
I agree with 22GT and BlakeTX. For a budget build, look at your stock parts. Replace things that are truly worn out, but the big goal should be to free up your suspension a bit, and reduce body roll. The Shelby drop, along with a 1" front swaybar and some better shocks will absolutely transform your ride quality. You might consider roller spring perches too. If you can afford them, Bilstein shocks are fantastic.
Another thing not to overlook is your alignment. With manual steering, you want 2-3 degrees of caster if you can get it. Power steering would like a bit more. This helps with high speed stability, as well as return-to-center, and gives you just a bit more camber when cornering, helping to brace the tire and keeping them flatter in a turn. Too much will make it harder to turn the wheel. For camber, anything between 0-1 degree of negative camber is fine; stay close to 0 for street use, but avoid positive camber (top of the tires tipped out). Adding more negative camber will make the car handle more aggressively, but also cause uneven tire wear and make it feel more wandery. You want about 1/8" toe-in so the wheels are exactly parallel when rolling down the road with both sides being pushed 'back' by road and wind resistance.
Coilover shocks are awesome, but considerably more expensive than upgraded stock suspension. Using 'made-up' percentages, I'll try to describe the difference:
Going from the stiff factory stuff with a bias-ply alignment to the changes described above will be a quantum leap. Perhaps 100% better driving experience, subjectively. Going from that to a good coilover setup might be another 10 or 15% improvement.
My car originally went around corners like a power boat. Tons of body roll, tires squealing, and even after the turn was over, I was still spinning the steering wheel to get it straightened back out. After the changes above, it handles like a new car, and just does what it's supposed to, even when driven aggressively. Ride quality improved significantly too, despite also going to stiffer, shorter 1" drop 620# springs (just to get the nose down more).