To cast off a stitch you have to have 2 of them, and you don't take them from the sts you've already done.
So after the k7, knit 2 more and lift the first one over, k1 and lift 1 over, k1, and cast off, until you've cast off 6 sts; count the sts you lift over, not how many you knit. Then you have 1 st on the needle from the cast cast off stitch and you knit 55 more. Then knit 2 more and cast off 6 and knit to the end of the row. Many seamless sweaters are done like this too, you'll have 3 sections and it may be better to have separate yarn for each one if you knit all the stitches as one row rather than finishing each section separately.
sue- knitting heretic