Because of the trim, I previously assumed that Janet's dress had a princess seam there. Upon further research, I think the trim is superficial because I can see bust darts on the sides. Bust darts usually accompany waist darts, too - which may be hiding under the trim.
McCall's #2718 has a sloper (shell) that will work great for this top style, but you'll still need to adapt the bottom of the dress into a retro A-line style. You'll also need to move the closing from the center front to the center back (and adjust your seam allowances accordingly).
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Basic sloper pattern (most pattern co. have one) |