If you lower your car very much, the strut will start bottoming out. To mitigate this, shorten the strut. It's easy and can be done with or without welding.
Overview
Remove the spring, then grind the spring perch free from the strut tube.
Cut the strut to the length you want. For a three-inch lowering job, cut the strut three inches.
Then place the spring perch where you want it and fasten securely. You can have it re-welded or use a commercial-grade shaft collar.
Cutting Strut Tube
Can a cheap tube cutter do the job? A pipe cutter might work but they're actually quite thick walled tubing so it might take a bit!
A good old drop saw (chop saw) does the trick on the tubes. It will hold the tube perfectly square and just cut it through.
Cut down, spring perch welds ground off
Ready to prime (use weld through primer to seal under where the spring seat will go)
Primed, now ready for fastening the spring seat
Fasten Lower Spring Seat
You can place the spring perch where you want it and fasten securely. You can have it re-welded or use a commercial-grade shaft collar.
See main article: Fastening Spring Seats