The difference between single and double flex is single flex has heim joints on one end of the control arms. Double flex has the heim joints on both ends. The arms are identical in length. You will have to get longer control arms to re-center the axle if you throw a 3" spacer on the 5" coil. You have no choice if you want it to be right and safe to drive on. I have researched ALL different combinations of lifts to get the maximum amount of lift with the maximum amount of articulation. Guess what, none of the combinations I did had spacers involved. I wonder why that is...hmmm...If you want to make a good lift, do it right and get the necessary equipment to begin with. Do like Biginuf did, special order some longer springs, and have control arms stretched. Any good welder can do it, and pretty cheap as well if you provide the parts. Or get the new 7" Coil-Over Skyking system that is coming out from Skyjacker. Lift kits and systems are not limited to what they come as. After all, YOU payed for it, make it the way you want it!
My .02