Connie, Math was always my worse subject. Why did I need it anyway, because my husband is my calculator. :-)
You will be fine with your rectangle all through the quilt pattern, because the squares used are all the same size, so if your rectangles are all the same size it equals out. But the quilt pattern also uses HSTs (a square made of two triangles sewn on the diagonal). So you would need to sew HRT (Half rectangle triangles). You could draw a diagonal corner to corner on your rectangle, and make a template of the triangle by adding a seam all around the three sides. You would have to do it twice-- once with the diagonal going corner to corner the long way of the rectangle, and again with the diagonal going corner to corner the short way of it. You could still cut strips WOF (width of fabric), but they would need to fit the size of your triangle templates. The finished Giant Swoon would have an overall stretched look, rather than being symmetrical.
Another solution is to just add small rectangles down the narrow side of your center block to get it to the correct dimensions. It means that when you add the next parts of the quilt pattern, the seams wouldn't match the next rows of squares, but that's ok. :-)
I guess it just depends on how many strips you've already cut for what you might like to decide to do! :-)