First of all, scrap the PDI 120% spring, it'll be a pain to install (likely), and it'll crack the front of your mechbox (a new reinforced mechbox, V2, can be had for $40) in a very short order. Don't know the fps you were hoping to get out of it, but get a Modify 90 spring from airsoftparts.com, it'll be easier to install, and make your mechbox happier too for longer. PDI has made crap inconsistant springs for the past years, I've dealt with a half dozen 150% springs, all ranged from 6 3/8" to 7", different metals and wire thicknesses, and have output between 350fps and 425fps. Go with Modify, and save yourself frustration and extra money to be spent for replacing more expensive parts.
A guide, lots out there, even videos, try youtube first, others here that are more familiar can link you with guides. I have a book for his, but also enough years with enough mechbox types and gun types under my belt that I don't need those references on my computer. I also know the inner workings on an intimate level, so it's easy for me to work my way around guns.
Best of luck, and enjoy your new puzzle!