I would sash them with 2.5" strips the put the squares so they are square against the ruler grid and trim the sashing. It will make for wonky sashing, but your blocks will look square and straight. Lots of blocks are sashed wonky with sashing as a design choice.
http://cjsews.blogspot.com/2010/04/t...y-sashing.html gives a great tutorial on this subject.