Hey, i think nasus is the best tank ever with shen, because nasus can allready tank and play offtank, First, you'll get doran shield, you'll sell it for buy Aegis or warmog.
After you'll buy ninja tabi, aegis, warmog, trinity, and now you can take Thornmail or Force of nature, and finish with an atma's impaler.
After this, you can sell aegis for buy a second warmog and finish with approximately 5.5K HP and 24X AD with a lot of armor point and magical resist

Now i've finish my first build, good game and have fun =))))