How patient are you and how clever are you?
I thought of the applique solution to but then I thought I wonder how difficult it would be to cut a template for the curved edge, to sew/add on all the corners to bring the block into line? This would give you a head ache I know, but also it would give you a sraight edge to work with. The blocks are so old, I would like to preserve as much of the originally intended design as possible. I am sure because of the mistake this is why they did not get sewn into a quilt in the first place.
I have loved seeing the pictures, do keep us up to speed with what you decide to do won't you!
Gal