When I made this large wall hanging (notice it is over a davenport!), I used light-weight interfacing plus Warm&Natural batting. By the time I had both the quilt top and the backing, it was a very heavy pull to quilt. When I sewed the backed t-shirts together (definitely straight stitch), I used 1/2 inch seam to make sure the two back-sides of seams touched for quilting. The 1/4 inch was too small to open up. Hope you have a quilting angel sitting on your shoulder while you complete your quilt as it is going to be tough. Not knowing what the quilt top is like, might I suggest you make a separate throw with the t-shirts? Been there, done this....aching shoulder and elbow for months.