I put a Kehin 39mm MK1 on my 400 a while back and it's been great, apart from I bought one with no choke circuit (Doh!) so starting is a bit of a trick from cold. Otherwise its all good after a bit of jetting work.
I posted a few threads about my lerning curve so search my posts and gallery
http://www.husaberg.org/gallery/v/Members/loz16h/ for the full story and pics.
There are 2 types of Kehin FRC - MK 1 & MK2 with lots of different configurations of mounting spigots. Mine was fairly easy to adapt to stock carp rubber. 39mm is generally agreed as most suitable for a 400. The MK2 comes with hot start button and TPS but I dunno much more than that about them.
I originally got the Motion Pro push pulltwistgrip and cables
http://www.motionpro.com/motorcycle/partno/01-0073/ and
http://www.motionpro.com/motorcycle/partno/10-0077 then I got a scotts damper and the cables were too short so I recently found out that Motion Pro part 10-0078 is almost 3" longer and I'd recomend that length even if you don't have a damper as the others are a bit on the short side anyway. The 10-0077 is about 29 1/2" or 30 " (which by chance, is the same length as the stock dellorto choke cable, but I digress).
I think these carbs are all setup for the fuel inlet on the left which puts it right in the way of the exhaust header. So the solution is to move it to the other side. Mine is blanked off on the left with a rubber washer and has a Sudco 2 into 1 thingy on the right side. I was lucky to have it already fited and don't know how tricky this is to do.
Another tip is; read up in the Doc for jetting advise but the short route is a jet kit from Taffy - recomended.
