I will follow the crowd, and add...
The FMX is a stout tranny is stock form, however there is little to no aftermarket support, and it is VERY heavy.
The C4 can be built very strong, is light, and has a great aftermarket if you need one. There is little to the swap from FMX to C4, however I am like the others, get a strong built AOD. You won't gain any weight from what you have now, and you will cruise comfortably on 3.50-3.70 gears. 3.89-4.11s are borderline too much if your engine isn't built to rev. They can make it feel more like you are driving a truck. Even though the OD would help it in cruising, if you don't need that much gear, then it is kind of a waste in the first couple gears anyway.
2 things:
TCI has a very bad rep for building trannys in quanity, not quality.
AOD trannys are weak in stock form (especially vs AODEs), so get it built right, not stock. AODEs are stronger, but the 'E' means that it requires computer input to function, so it is not the best choice.
Good luck
Dave