i usually lay 2x6 flat on the bottom of the header, with the extra two inches toward the inside. depending on how you want to do it you can either continue with the 16 o.c. or just put insulation in the pocket. you need insulation regardless but you only need every two feet for the drywall. if you just want to you can also put a strip of half inch osb or similar between. alot of choices .
Build a "box" header with a 2"x6" on top and bottom, a 2"x8" nailed flush to the the outer edges. Don't worry about the gap in the middle as KC said unless the span/load requires something solid. That (along with your double top plate) will net you a header height of roughly 83".
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