I would try blocking the blocks rather than trimming. As someone said, measure the size the blocks should be, dampen and stretch to that size. Pin the edges and allow to dry. You can get a "board" that is for pressing but has a grid on it that you can use to both measure and pin to. Pin all the way around, close together and allow to dry or press dry. Once you have blocked them you can trim or sash as necessary.
They look like they will be well worth the effort.