Question - What is your final goal for this meat - jerky, pulled beef, or steaks?? My 2-cents worth is that flank steak should be treated as steak, and only cooked to 125*-135* depending on how rare you like your meat. I think a steak always benefits from a reverse sear - giving it a little smoke until meat reaches 105* or so, and then putting it on the grill to finish it up. Just my thoughts.
In southern California we use a lot of flank steak in our Carne Asada tacos. I would cut them down to 1.5# to 2# steaks. Marinate in a can of coke with some cirtus like oranges and lemons and thrown in some chopped cilantro. After a good overnight marinade, rinse and reverse sear as described by olysmokes. Slice thin and against the grain. In a warm tortilla with pico de gallo, some grilled green onions, and a dollop of guacamole or sour cream...MMMM
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