I havent seen any aftermarket a-arms that are stock width. You could cut off the ball joints and get a tie rod end for a s10 and slide it through the hole thats in your factory arms and put a nut on both sides. Or you can bite the bullet and go +1 or +2 and get longer tie rods. You dont have to get shocks for any standard travel a-arms. I cant say enough good things about my albas. Cartwheeled the quad a few weeks ago and bent rim and spindle but a-arms are straight as an arrow.
+1 a-arms are primarily for gncc racers. The small amount of added width will make your quad handle alittle better. As far as jumping goes it will have no effect on that. That is all about your suspension setup. Stock 400ex pogo sticks are terrible for almost everything. You wont need longer brake lines. You can even do a +2 without new lines but they will be alittle tight. Unless you get LT a-arms you can run your stock shocks. And you will need longer tie rods. But thats it.
Your stock steering stem will work. All the +1 are really expensive last i saw. JB and Houser i believe were the only ones that made them. And alot of 400ex stuff has been discontinued through those companys because no one really races 400s anymore(Besides me lol) I have to custom make alot of things work in alot of cases. But the reliability of the 400 is so worth all the hassle.