hi, I just purchased a magazine with instructions for a T-shirt quilt - I also wish to try my hand at one. Their instructions:
"look for interfacing brand names such as So Sheer or Fusi-Knit. The greatest stretch of most T-shirts goes around the body (crosswise). To stabilize the knit fabric, place the interfacing so its stretch goes the opposite the T-shirt's stretchiest direction (usually this means putting the interfacing's greatest stretch running lengthwise), then fuse in place.
I hope this helps a little. :)