Ford F-150

As long as the free standing height is the same and the springs are just stiffer. You don't want to change unloaded ride height because that will change the front suspension geometry. Even when the numbers are set correctly on the alignment computer, that only pertains to a truck that's standing still. You'll still get lots of bad tire wear as the geometry goes through its changes as the truck goes up and down on bumpy roads. Start by measuring the front height at some convenient point on each side such as the side marker lights with the old springs and again with the new ones. If it's pretty close with the new ones, there should be no increased tire wear over what is normal.

Ahh. I doubt you'll break one. There are rubber stops that the rear axle will hit when the suspension is bottomed out. Those are often mangled from being hit so often but nothing happens to the springs. The concern to me is the handling characteristics from the reduced ride height but three miles isn't much. I have an old rusty '88 Grand Caravan that has been grossly overloaded many times. After every trip it seems to sit higher than normal on the back. No one can figure that out and I'm a suspension and alignment expert. I hauled 50 sheets of sheet rock 90 miles, and 33 12 foot long 2x8s over 60 miles. The rear suspension was bottomed out after I had 15 of them loaded. The springs still survived. For what you're doing you might consider helper springs that bolt onto the bottoms of your current springs. They won't do much when the truck isn't loaded but they will help maintain proper ride height when it is. Also keep in mind most trucks and minivans have height-sensing proportioning valves in their brake hydraulic systems. A proportioning valve is used on every vehicle to limit fluid pressure to the rear brakes to prevent wheel lockup. That valve is fairly easy to design for a car, but trucks will have such a wide range of weight distribution between empty and fully loaded that one proportioning valve can't suit all conditions. When your truck is heavily loaded, more braking pressure will be sent to the rear brakes to help it stop. When you change the springs to change the ride height, that height-sensing valve won't know there is more weight in the back so it will reduce how hard the rear brakes apply. Again, you might not notice that in three miles, but there's always the "what if" principle. What if a kid runs out onto the road and I can't stop?