Yep just cut the milk gallon in half! But like skunk said about the flushing to prevent heat stress and root bounding....
a) Keep an eye on the temp in your grow room, I didn't notice any heat stress in the plants but could be wrong. Temp should be from 75-80 degrees fahrenheit.
b) The root bounding problem will take care of itself once you transplant. Due to the size of your plant I estimate the roots have covered between 40-60% of the volume of the milk container, this means that the roots probably aren't bound together yet and the plant will most likely respond very well to the transplant, but keep in mind, the bigger the pot, the bigger the plant! The if you give one of the plants a big pot are the rate they are growing you would probably only have enough space to support one or two large plants during flowering, I just ran into this same problem, tho my problem worked itself out as I had 4 plants growing nicely and 2 ended up being male! anyway, Good luck, plants are looking great!
PS What kind of LST are you doing? Topping? Bending? Tieing?