I'd rather have a larger set up that my snakes will grow into than have too small of tubs in 6 months.
A 400 gram snake is on the edge of needing a larger space than a 12qt tub offers.
As Elizabeth mentions, just clutter it up a bit with branches and or some fake vines and the snake will be fine.
I have a few 500 gram snakes in 34qt tubs and they seem to love the space.