That pattern would finish to 2.5x2.5 inches for each *increment * of your grid. Around the perimeter, add in the 1/4 inch for perimeter seam allowances. You could measure each *increment* of your grid.
With five *increments of your grid* (some with one or more seams within that grid-increment) you have lots of seams with room for the *width of the thread stitching line* to be off.
With luck, you could frog-stitch one long seam perhaps and add in 1/16" inch beyond the point(s) in that grid... and/or figure on stretching a bit when matching seams for the adjoining blocks. I.E. if you are matching points every 2.5 inches sewing one block side to the next, you would be stretching perhaps 1/32" of this one block to match the next for that increment? Its fabric, you can do that LOL.
You might also dampen just the seam on the back with a sponge or wet washcloth and press with the tip of the iron, trying to get that seam *flatter* (dont stretch when wet tho)... you might gain 1/16" of an inch in a couple places.
Overall, in the end... you can probably stretch that 12 3/4" measurement with all the seams in that block, to match a 13" strips, so, really, I dont think you need to take any stitching out...I really would not rip anything; the blocks looks BEAUTIFUL ! .... I think you can *make it fit* that extra 1/4 inch.... 
Last edited by Sheepshed; 04-14-2012 at 11:21 AM.