I figured out the seperating part- it was actually really easy. I did what Kati did (moving the markers so there are more stitches in the back), although I moved the markers five stitches on each side.
I worked and bound off the back stitches first and then worked the three inches in the pattern. I found doing the three inches made the front too tall, so I went back about 1 1/1 inches and bound off there. I worked the straps for 12 inches and I'm going to do the three-needle bind off on the back. I'm going to try on the tank and figure out where I want the straps to be before I connect the straps.
Once that's done I'm going to block it and it'll be completed! *YAY!*