I've seen many tshirt quilts done, never made one myself. But, I'd put the stabilizer on, cut the blocks out.
The take fabric and lay it out, arrange the blocks on it like you're using it for the sashing and see what you think. Audition several different fabrics. If the ones you bought don't work use them in something else. Take your blocks to your local LGS, lay out different bolts and see what works.