Contact Jim at PPI
www.mustangparts.com
He'll set you up with the right shocks (and even springs) to get the car to ride right.
Look in the archives in this forum, as I always recommend the H&R or Eibach 450-580 progressive rate. AWESOME street spring.
p.s. My 425# coilovers translate to (depending on who you ask) 1000# to 1700# rates. VERY stiff, but this is for mostly a track car, and with low friction delrin bushings...