Is there any reason this kit wouldn't work with a big bearing 9inch from a '78 bronco? (minus fabbing up new hard lines to the flex hose)

No reason we can think of, other than the included hard lines. The kit includes brackets and spacers for the Ford 3/8 bolt- small bearing, 3/8 bolt- large bearing, and 1/2 bolt- large bearing 9 housings.