A long time ago I went to a workshop and the instructor (forgot her name) showed us how she worked with too small blocks to fit. She cut freezer paper the exact size of the block she wanted, then iron the too small block on it. Then she cut the paper from the block leaving the edges. She then had a guide to sew blocks together, fudging the seam allowance. Of course the block has to be slightly smaller and have enough for at least a 1/8 seam of fabric. I have sewn extra fabric to the block to fill in the seam allowance I needed. I have used the freezer paper way when I have blocks from exchanges or blocks made by others that have to go in a quilt.