Yeah I wouldn't go for some exotic 4.0 gear if the 3.9 is readily available. Anyone that's done a gear swap will tell you that it really changes the way the car drives, acceleration wise. My gear swap was only 7% or so. Twice that is going to be crazy. I haven't crunched the numbers, but I'll bet you'll have to hit 3rd to make it to 60mph. Keep the old gears if you do go FI. As long as it's Nissan parts, you shouldn't have a problem with quality.