If I'm understanding correctly, then if you just knit on the purl side and purl on the knit side, the letters and numbers will show up, fine. The one thing to be careful of, is that knitted stitches aren't square, and so you want to use this kind of graph paper to make it look right: http://incompetech.com/graphpaper/asymmetric/
If you use normal graph paper and on the paper, the letters look the way you want them to, when you actually knit it, the letters will be wider and shorter than you want. But if you block out the letters on this paper, it will come out right. I would do it on a small swatch to make sure you're happy with the results before actually knitting it into the blanket. Oh, and just allow one row per st.