Hi, the answer to your question is "who knows"???? Not being a wize azz, but until you get all the major lift parts on and mounted, you really will not know what length shock you will need. Same goes for the limiting straps, if you plan on using them (highly recommended).
I am also planing another lift, and of coarse I like to mix parts from different manufacturers, so I am sort of in the same predicament. I had a small lift on my Dodge, about 2-1/2" using a poly spacer. Now I am going with a Skyjacker 4-1/2" lift with new rear leaf springs. I already have the FabTech dual shock mount mounted and will leave it on even though the kit comes with a dual shock mount kit, the FabTech piece is much nicer.
I figure I will run 8" stroke shocks up front and 10" in the rear, this figure is based on what I currently have. Hope it works, since I already purchased 4-FOX front shocks and 2-FOX 10" rear shocks.
I can pretty much go ahead and measure my trucks static shock length, and then compensate for the added lift, since I have the dual shock mount already mounted. You really can't accurately do anything till you have that in place. Good luck with the lift and let everyone know the outcome.