Attach 3 supports at the top using the remaining 15" pieces. The middle support needs to centered at 63.5" to accommodate the height of one specific bike. The top support is flush with the top of the 77" vertical boards, but its center is is at 75.5". The lower support is hung at 51.5" in the center, but you can get away with hanging this wherever you feel like, or adding addition supports in the middle of the structure.
Be sure to pre-drill holes to hang the hooks before attaching the top two support pieces.