No membership needed for AWRCS. If your under 18 you will need a parent there with you to be able to race. Fox floats should be just fine for xc racing if they are set up properly. A steering stabilizer is good but not needed to try a race. Most likely the over bored 400 will be just fine as well as the other mods that you mentioned.
AWRCS #484 QXC Senior class - yeah i'm old but i can still ride!
2000 400ex , dg pipe , k&n , ac nerfs and front bumper , burgard +2+1 a-arms, armidillo rear skid , rock beadlocks all around , kenda klaw xc's front and rear , 450r front shocks , GT Thunder front and rear springs and xc link , tusk braided front brake lines and clamps , moose case saver , streamline steering stabilizer , renthal bars , much more to come!