You could pick up stitches for a neckband, but you could also start the split for the V a couple inches higher or wherever you want it to be.
Oh, I guess you're essentially done. Then try the neckband, use a smaller needle than you knit the sweater with. Unless the instructions you found are for a sweater with the same measurements, gauge and yarn weight, the sts you would pick up may not match. A good way to do it is to pick up 3 sts for every 4 along the neckline and stitch for stitch across the back of the neck, making sure you get a stitch number that's divisible by 4 for the 2x2 rib. That works regardless of how big the neckline is, or your gauge.
sue- knitting heretic