Movie armor had raised round tabs an a hook to secure it to the inner chest. Either you recreate that or remove some return edge on the outer chest of your armor bringing it closer to the inner chest making it possible to use velco or similar.
As for the ab section, echoing above assemble the armor first. You can always trim more from the center back if you need to later. As Glen mentioned the entire rear seam is covered regardless.
Keep up the great work.