IF I understood what you are saying:
I think I would get all the blocks to be the same size before trying to put sashing BETWEEN them
To get them to be the same size, I think I might try one of the following:
1) add a fourth row to all (or some) of the blocks, then trim/square to a uniform size
2) trim all the blocks you have now and trim/square them to a uniform size
If you want the blocks to line up and not be a nightmare to work with, I don't see any way around some trimming of the units to a uniform size.