Now I understand what you mean, personally for me I think I would take of the sashing on the last column and put the tri´s directly on to the blocks, however whether to sew them first in a line and then do it is difficult, maybe they might not fit right and you would have to cut to size to fit.
I won´t have that problem (no Tris) but I think you will have to pin them all together and then try up against the blocks to see if they fit. Maybe Lesley could tell you
Hope that helps
Elle