I just installed a set of Intrax springs in a buddy's 1990 Supercoupe. The front springs were removed by unbolting the middle bolt, then I lifted the car in the air with the jack, and it relieved the tension on the springs, so I just had to unbolt everything else and take the shocks / springs out.
As for the springs settling in .. we noticed that it did drop about 1 inch initially, and then it took about 3 weeks before it dropped even with the rear end. But after we jacked the car back up to look at something else, it was at 1 inch again. Its been a couple months now, and I think the front end settled down by now - but the springs are made for the V8 afterall, not the SC, not sure of the weight difference.
I had installed a set of Intrax 2" springs when I first bought my 1994 Tbird back in 1997 .. 10 years later, they are about 3 inches lower than the stock height, so they settled in, and then some.