maybe add "coping strips" to the blocks. Then square them up all to a slightly larger size than the larger blocks. the coping strip size will be different from block to block but with the sashing you have planned it will look great.
I have used coping strips on lotto blocks, that never seem to be the same size.
The book SETTING SOLUTIONS by Sharyn Craig shows this idea, plus lots of other ideas for working with blocks of different sizes. It's a great book that I keep going back to whenever I'm stuck and need some new inspirations.