Those measurements are all pretty damn close. I'm assuming head pipe means steering stem, in which case it seems close enough to press the X stem into the L lower clamp, heck even a 10mm difference isn't that bad, I'll try to get my hands on a lower clamp and stem from an X to see if it will work. Someone else should give it a go too, I think the guy I know only runs 450s.
THe suspension seems good to me, this is my first "dirtbike" so I've got nothing to compare it too. I just want stiffer dampening because of my portly stature.