The ns250 MC11 pistons are not the same as the ns400 ones (that are not available anymore anyway). Although the stroke on the crank is the same 400/3 does not equall 250/2.
The con rods are also different.
There are a couple of choices of other pistons you could use and for the money you are going to have to spend, some much more interesting (=more powerfull) options available. PM me for details.