It looks as though the sides of B do not form a straight line, but are at an angle back from the center seam. Is that right, or is that edge of the tarp a straight line, making it a true rectangle?
The point of the beak on the edges will probably stick out beyond the edge, but just cut it off. If you made the tarp wider than the plan, the beaks might even fit better, but might not go all the way to the edge of the tarp. When you sew on the first edge of the beak, the center seam and unsewn edge of the beak will be at an odd angle to the unsewn edge of the tarp. Both seam and edge will straighten out when you sew them together. When you sew the first edge of the beak on, start (or stop) the stitch at the edge of the center seam of the tarp. Do not sew over either the seam of the tarp or the center seam of the beak. The beak seam needs to be able to swivle up to match the center seam before you attach the other edge of the beak. I think you will see this when you align edge A and B. The center line of the beak will not be parallel to the center line of the tarp. This confusing explanation will be more clear when you start to line up the edges of tarp and beak! Try pinning the beak on before you sew, maybe that will answer your question.
If your anticipation of problem is correct, just square off the tarp edges. I think otherwise, the beaks might droop too much.