I think if you were to do it you have the right ideas here. I realize I've been coming off pretty negative sorry about that
I think if u want to get the most out of your setup outside you should start the plants inside awhile before it gets the time u want to put them outside and then when they do get outside they'll grow alot faster with more foliage to absorb light. I think a plant may have a hard time using a whole 400 gallons in a grow season if you didnt give them a little bit of a head start I may be wrong but that's alot of space for the roots to take up if you were to just start your seeds in April or may when you would actually put them in your greenhouse
A question to the experienced growers..
What do you think would happen if you started seeds in say December, and veg them inside until it's time the next year to plant outside. By that time the plants would probably be like 3 feet already and when you put them outside they would still have a whole season to veg a whole lot more and flower. I mean they would be a couple feet while the plants other people are starting at that time are a few inches.. Do you think this would create a monster plant? Is there a limit to how big a plant will actually get if the season length isn't an issue?