Technically you should replace the main shaft. It's not the nut nor the gear that really wears, it's the shaft itself. In '99 or so New Venture updated the shaft with longer threads. The problem is that the nut does not engage fully on the threads when tight. An updated shaft has threads all the way to the splines (no gap).
You can install a new gear and nut but due to the wear in the shaft it's really only a temporary fix and will eventually work it's way loose again. I haven't read your build thread so I don't know what you have for a motor in front of that thing. The 5th gear nut problem usually occurs in heavy towing applications and lugging in 5th gear. The cummins trucks had big issues with this because that motor can really lug away in 5th gear, especially with the common 3.54 gear set, not to mention the many ways to get more power out of them on the cheap. The issue was less problematic on GM's because gas motors really don't have that huge torque down low nor did the 6.5 so the user generally had to shift to a lower gear. SO, the correct thing to do would probably be replace the shaft, gear, and nut with the updated version. Since this is probably going in a K5 I might run it like it is and cross my fingers. You are way under the GVW of the NV4500, you most likely won't be towing 10k lb trailers, and that shaft is a pricey fix. one could argue big tires, incorrectly geared in 5th gear would have similar effects to a trailer though.
Hope that helps,
Mike
