I definitely recommend using fusible tricot for the fusible interfacing. This is a soft knit fusible that doesn't make the t-shirt fabric too stiff. JoAnn's has small amounts of it but, for a king size quilt, it would be more economical to buy it online from a drapery supplier. This is where I bought mine:
http://www.bblackandsons.com
and here is a link to the product:
http://www.bblackandsons.com/tricot-...-60-p-302.html