When I put my P heads on I used the TFS spring upgrade kit for iron heads. It is part #TFS-2500100. It includes everything that you should need, springs, retainers (they are specific to exhaust and intake), locks, seals, shims and an intalled height checker ( it is 1.8" long and you use it to determine if you need shims and how many so that your spring is at the proper installed height)
Pretty easy to do if you have basic mechanical knowledge. Heck even I did it and my car runs and doesn't use oil. At least I got that right.