Yes you will need to order the P18 spring which is the same spring offered for the Predom. Type in Lucznik predom pistol tune, or Konceptus. You can choose to buy the full kit or the mainspring on its own.
Your engineer will need to make a Delrin guide rod or modify the one provided by Konceptus (if you order the full kit) to fit your Walthers.
I would go for the full kit. Let me know how you get on.
Had planned to charge you minimally in order to try something slightly different but doubt ill go into production since acquiring a teaching post.