One of the long edges of the rectangle will be seamed to the Sleeve/Top piece. So one stitch on that edge will be maintained in stockinette stitch. The other edge will form the lower edge of the cardigan. Two stitches on this edge will be maintained in stockinette so that this edge will also roll slightly toward the body. I swatched a couple of variations here, and I preferred the 2 stitch version.
The body is knit from side to side, so if I use a stitch pattern with vertical lines, these lines are going to end up looking like horizontal stripes - not so flattering in a bulky, loose-fitting cardigan. A horizontal stripe pattern will turn into vertical stripes - much more flattering.
I swatched a couple of variations of stripes made with alternating sections of stockinette and reverse stockinette stitch. Any more than 2 rows of each, and the work really lost structure and started to sag. So, the body stitch pattern will be:
Row 1 - knit
Row 2 - purl
Row 3 - purl
Row 4 - knit
Repeat rows 1-4 until desired length is met. And that's going to be a while - this rectangle will likely need to be about 70 inches long!
Now, here's a little trick to make this a bit more tolerable for me. I'm going to use the trick play technique, just with two balls of the same color instead of alternating colors. No purling!