i was wondering what is required as far as modification to put a 400ex swingarm on my 250x frame

 

Unless you have the entire rearend of the 400EX, this swap is not cost effective for what small gain you would get. Major machining is required along with welding linkage mounts. These kinds of mods to 250X and 300EX quads usually only go down when using 450R parts,..not 400EX.
You would be better off in the pocketbook in the long run by finding an aftermarket swingarm that directly replaces yours.

 

what about a 250r rear swap i want to gain about 2"in the swingarm

 

Same thing with the 250R,..it requires major fab work and welding on the frame.
There is no bolton swap for a 250X/300EX swingarm.

The old 350X swingarms are the only thing that come close to being a direct fit but they require alot of work to install to. Lots of spacers and bushings need to be made, along with a brake hub swap, and having to use the 350X shock, etc,etc. It's hardly a bolt on and ride project. They are about 1 1/2" longer than the 300EX/250X swingarms.