If you haven't already bought it, don't buy the Vortech unit. I have the FMU (different from the S-FMU) and it hardly works. I really only see an increase of fuel pressure in 3rd-5th gear. I don't know whats wrong with it, and I've done a lot of testing on it and can't figure it out. I 'think' I have the 8:1 disk in there right now, but I can't figure it out since they aren't even labeled!
If I were you, I would get the Cartech FMU. It has vaccum FP control, as well as on boost settings. It is also a heck of a lot easier to adjust since you don't have to change the disks.
If you have the Vortech, then just use it I suppose, as maybe mine is a lemon. MOST guys running 5 lbs use the 6:1 plate, though I have seen some running the 4:1. If you have the fuel pump, and this also depends on your baseline fuel pressure, you will probably be better off running the 4:1. No fuel pump, run the 6:1 or 8:1.