A quick word about rear springs: if you're going to convert, just get springs from '07-'14 Mustangs. I use '11-'14 GT springs and couldn't be happier. They cost me $20, were basically brand new, and had a 3/4" drop over my stock Grand Marquis springs. Any air bag replacement springs will also sit way too high.

Now for a rear coilover setup: it is actually possible and has been done, HOWEVER, the guy who did it, his Crown Vic is a road race only car. He uses a coilover rear shock in place of the stock rear shock with a modified upper mount and no springs sitting between the axle & frame.