Razor, I see no advantages of putting a 700R4, a 4L60,80, or E in a gn. The gearing is not optimum for the GN's power band, plus there are inherant design items that limit the strength of those trans. We have cars pushing well over 1000HP on a built 200R4. You may ask Chris at CK, he is starting to do some stuff with those trans', maybe he has some other idea's that I am not aware of.
I'd love to bolt a 4L80E into my GN.
I've spent alot of time playing with the 4L80E in my truck, and just love it. Soft short shifting at light loads, and then firm or hard if you wanted shifts at WOT. RPM adjustable shifting for each gear. They are sweet. Even better would be using a PCM that was compatible with the GN, and then being able to do torque management during shifts, where you can retard the timing DURING the shift to take more load off the tranny as it shifts gears. The 80E's gearing is close to the GN's. Down side is weight. It's a heavy tranny but if it's bullet proof then that's work for me.