Wanda over at Exuberant Color has been doing something similar (not with tshirts but with nonconforming blocks) - you can follow her tutorial here (start at the bottom for part1):
http://exuberantcolor.blogspot.com/s...ilt%20tutorial
Partial seams can be your friend:
http://quilting.about.com/od/piecing...tial_seams.htm
IMHO, it makes a much more interesting quilt than a straight grid layout.
Or you can use coping strips/sashing to build up each block so they are all the same size.