If you can match the yarn in the blanket to a heavier yarn and use size 8 and 9 needles, that will work. Or use the sport weight with a size 7 and 8 needle, sometimes that works, especially if you're a tight knitter. If we knew a little more about the pattern (do you have a link to it or a picture?) we could advise you a little better. For instance, if it's a top down raglan, you can just keep doing the increase rows until the back sts measure half the finished chest size you need. That's the easiest to adjust, but if it's not that style, then there's more that needs to be done.
sue- knitting heretic