PM Sent.
As for fitting the swingarm, they're a common swap on XT/TT/SR500s.
XT/TT/SR500 swingarms are interchangeable with XS650 swingarms and vice versa.
It's my understanding that the DR400 swingarm is about 3mm narrower at the pivot so it needs two 1.5mm thrust washers to take up the space.
The axle slots may need to be opened from their current 17mm to 20mm if using a XS rear wheel, again easily done with even a hand file.
The pictures show just how similar in dimensions it is to a stock XS650 swingarm.
Pretty minimal work required to make it fit.