Don't bother. The smart thing to do is replace it with an AX-15. You can get them brand new from Advanced Adapters for $1500, and they are a direct replacement for the NV3500 which is a much more expensive transmission and harder to find parts for.
In regards to rebuilding a transmission though, that is not something you want to attempt unless you've done it before. Definitely not an easy thing to do, and it requires lots of special tools (and skills).
Disclaimer: Some will chime in here (I know they are lurking) and chide me for putting down the NV3500 as I always do. I'm not putting it down, that's not what this is. This is me stating a 100% fact. The AX-15 is equally as good as the NV3500, it's much cheaper to buy a new one (and buy parts for as well), and much more readily available. If you have a perfectly working NV3500, keep it. However, if you need a new NV3500 or need to rebuild it, you'd be stupid not to replace it with an AX-15.
