1) It's not "each harvest." It's "whenever you have over X bushels in the warehouse." If it were "each harvest," that would cause all kinds of problems with food not being transferred because it didn't quite get there in time.
2) Bearing that in mind, you would set up an automatic caravan order to sell to <destination region>, with the threshold at 400, and the amount being something high, like 1000. If you want to just give the food away, you can now set the sell price to 0, or you can set it to whatever you can see that <destination region>'s offering to buy for.