Somewhere i read that KDX 250/400 swingarms are the same as the KLX too.
I think DR/SP500/400 swingarm is also a good option. They are 508mm from pivot to center of axle slot and 393mm from pivot to shock mount. The DR400, SP500 and DR500S are all box section steel and the DR500 X,Z,D 'enduro models' (mostly found in USA/Can but a few made it to other countries) are all alloy and the same dimensions as the steel version.