I think most Nasus players prefer to build tankier than what a Muramana can do for you. They used to build Trinity Force for the proc but it also gave a little bit of tankiness/MS, and now you build the IBG instead for your main 'damage' item. Most of it really comes from your Q build up.
There was one guy who added me on LoL when I was just starting out and he taught me how to play Nasus really well by spectating some of my games and coaching me. Maybe if you're lucky he'll jump into this topic all incognito and give you some knowledge.
"When you want to be successful as bad as you want to breathe, then you'll be successful!"