I don't know where your manifold crack is located, but you might try to patch it with Rutland furnace cement, available for less than 5 bucks at places like Home Depot. I used it to patch the cast iron elbow to exhaust manifold joint--elbow provides manifold heat to the carbs on the ram tubes on a Chrysler 300G with a 413. Sealed the crack, and after 2 years there's still no exhaust leak. The guy who told me about this trick also said that if you tried to use it as a "gasket" between two surfaces, you would have to use brute force or a torch to break the seal. Sounded good enough for me.This stuff is rated up to 2000 degrees; It might not work in your application but it never hurts to try the cheap way first.