6an should be fine.. understand that when you
drive up to the lanes and get heat going its
building pressure so as you blast down the track
you are shaking that fuel building pressure so
it will take a bit longer to get it going into
vac(even with a vent)
EDIT
A rollover valve has a pretty small hole in them
so that would be the restriction point.. if you
dont want a roll over you can run the vent line
from side to side then down low in the chassis/frame
so its lower than the tank