Is it all down, down up, or does it matter? Like in "Your Gonna Go Far Kid" and "Cries in Vain". I try to do down up but I can't seem to hit the strings all at the same time.
Listen to CKY.
Metallics pretty much uses all downstrokes which I think is easier with palm mutes, but the speed is tough
It usually sounds better and ends up being easier if you do all downstrokes.

If the song is really fast however, you might want to try to use the 2 note version of the power chord and alternate pick it.
Down strokes are best - work on playing it faster by practising at different speeds, slowly getting faster until you get to the correct speed. It can take time but it's worth it.

      I dont know if this is what your looking for but some bands like it one way or the other. Metallica plays a lot of **** all downstrokes which is pretty tough at the speeds they play but when you have hetfield as a frontman it not that tough. Other bands like Lamb of God do everything alternate picking, even riffs that would sound good all downpicked. But they have practiced making the upstroke just as good as the downstroke. Jeff Loomis is another guitar player who is extremely good at alternate picking.