I did the Shelby 1 inch drop on my 70 and I have eibach 1 inch progressive lowering springs. I can lift the front end up and nothing falls out. I had to use a spring compressor to put the springs in still so I don't see why you should run into any problems. I guess you can use the eurothane spring spacers(mustangs plus has them i think) to keep tention if you are worried about them falling out still but I really doubt you need it. Note: the shelby drop will lower your car. Mine with the springs is about 3-4 inches lower than stock.
Kevin