Cast iron exhaust manifold can be repaired by broadcast heating the entire unit and brazing with an acetylene torch. An experienced welder at a local weld shop can do this repair with lasting results if it is done properly. Depending on the year and displacement of the motor you may be able to find an OEM replacement and/or one from a scrap yard. If you plan to have it brazed, it will have to be removed from the block for prep and heating. Good luck with your FXXX repairs.