Casey,
Yes indeed, the 77 F-2 Super Sport CAME with dual discs, it's actually why I bought my first one of them. From your pics I can only see the speedo cable on your right side. Is yours missing the entire rotor / caliper / hanger / and brake line? Any chance of going back to the owner and inquiring as to the whereabouts?
After having hot rods with minimal brakes, and average vehicles with killer brakes, I'd give up the horse power in trade any day.
But hey, you've got a great project going. Good luck.