Alright, here are the mods (off the top of my head) that my brother has done to his 400ex.

JE 13:1 Piston (larger bore to make it a 426)
Heavy duty connecting rod
Hot Cams stage 2 camshaft.
Keihin 39mm carburetor
Curtis Sparks X-6 full system exhaust
White Bros CDI box (10,500 rpm rev limiter with advanced timing curve)
And he is running 110 (i think) octane race fuel

Let me tell you, this thing is FAST. It is lowered for flat track racing and when you nail it, the front end just wants to go UP unless you lean clean forward to hold it down. It's nuts! 1st and 2nd gear are gone in a flash, that's how fast the engine raps. If you want your 400ex to be fast, I highly recommend this setup. HUGE difference! I can't even begin to explain how quick it is. His friend, who helped build the engine, has a 440ex and he says it runs harder than his 440 does.

 

Kick @$$[img]i/expressions/face-icon-small-smile.gif[/img][img]i/expressions/face-icon-small-wink.gif[/img][img]i/expressions/face-icon-small-happy.gif[/img][img]i/expressions/face-icon-small-tongue.gif[/img][img]i/expressions/face-icon-small-cool.gif[/img]

 

Ive got a 426ex also and I race flattrack also with a Ross Piston and a 11:1, Hot Cam ect..... Im lovin it!!!