My research suggests a 5 inch rim, takes significant machining to align rear sprocket to front. Consider a wheel off an rs250 or for cheapness a cbr600 (I think an f3 model?).

You would want to change the front rim too or you will 'over tyre' the bike. I won't go into what it means it'll start an argument about geometry again.

I'm new to the nsr's so my response is really based on heresay and knowledge of 400's.