dunno if itz possible, but i think they should make a reverse part u can buy. like u can buy a 2x4 unlocker 4 da wolverine.

 

If you have the money, skip the EX. Once the 450r comes out, ALL the companies will have surpassed the 400EX. The Z and KFX don't have MUCH more power, but it is a difference, and since you need reverse (same here), that or the Rappy seem to fit you pretty well.

 

You can buy a kit for about 700 dollars in dirtwheels which will make a 300ex to a 330ex and it will kick the crap out of a 400ex no doubt.