Preload occurs when you tighten the
suspension points that have the rubber or urethane bushings with the wheels hanging down and not touching the ground. If you tighten the bolts then, the zero resting point for the
suspension will always with the wheels hanging down, This will cause the bushings to have a twist in them when the car is setting with the wheels on the ground. The zero resting point is supposed to be when the wheels are positioned as if they are on the ground.
Put the jackstands under the rear axles housing and let the weight of the car rest on them. For the front of the car, the jackstands should be as close to the wheel as possible without causing interference with what you are working on.
If you intend to own and drive a modified Mustang, a good floor jack and 4 good jackstands are a must have item. That is a basic tool item along with quality American and metric size wrenches. You can very easily have more money in tools and shop supplies than in the car. Cars will come and go, but good tools are a lifetime investment Some of the Snap-On Tools that I have are 50 years old and still work better than any of the cheap Chinese junk you get now days. They have stuck around and been useful longer that my first wife, two kids and 10 different cars.
Harbor Freight jackstands are as good as any, and cheaper than most other places. I have some and am pleased with them.
I don't think that I would buy a floor jack from them unless I had several friends who had good experiences with their Harbor Freight floor jack. The floor jack I currently have came from Sam's, but I never crawl under the car without one of more jackstands in place.