You don't join the 330/340 fleet - you join the 330 or 340 fleet (or possibly 320 in your case). Then at some point in the future (possibly 12 months, possibly longer) you will CCQ to one of the others and eventually MFF either 320/330 or 330/340.
Although you have some input into this, at the end of the day they put you where they need you and at the moment that is likely to be on the 320 if you have that rating.