The first four gears hold up pretty good. Towing in 5th with 600+ ft/lbs tends to kill them quickly. They are a decent transmission, but not what many crack them up to be. The work a 12 valve can do is more than the NV4500 can handle in the long run. NV4500 prices are pretty outrageous and there's a lot of bad info out there about them. If you find an NV4500 setup for cheap or it's the simplest to mate to your t-case then it may be the way to fly.
If your 12 valve really is tuned right, making big numbers you can spit 5th gear off in no time if you try.
My '98 dodge 12valve nv4500 has 193,000 miles on it and is going strong. It is tuned to well over 700 ftlbs torque and so far no problems. If you are just going for hp then yes you will hit a wall for what a stock nv4500 can handle but there are companies out there that build some pretty tough aftermarket units. If you are going to be towing I think you will run into a problem with egts before you find the hp limit on the nv4500. But you do need to be carefull in 5th like averagef250 said.