Contact Walker Evans directly in Riverside. They are the only ones who can do the Needle Conversion. Other shops can do re-valves or add duel rate springs, but do to machine work & exclusive parts only Walker can do the Needle part of the conversion.
On a side note be very detailed on where you ride. What you have done as far as accessories. Be honest about you & your passenger weight. Print out a few pictures of the car for reference. What I did is I 4 corner scaled our XP4. This gave them a better detailed weights, but was not nessiearly.
Most guys I previously spoke to said they got 300 over 350 springs, front & back. I did 325 over 350 and have been very happy.