A picture would be helpful in making a suggestion. JB weld may work. Drill, tap and plug may work. Drill tap and install a small plate to cover the hole may work. Depends on the size, location and proximity to any moving parts may determine the repair. I would think this is a cast iron manifold. If, by slight chance it is the FE series engine it should not be that much problem finding a replacement but my guess is it is the big truck engine you are speaking of.
I would suggest you post over on the engine forums. Good luck.