What year is your car? If it's a later model car, replace it with a manifold
from a 1980 car. They made them with a reinforcement in the area where I'm
betting your crack is (can't remember the technical name - the spot where all
three exhaust leads meet). I had the same problem with my 1979 B. I bought
a used manifold for $50.00, ran a die on the studs, cleaned it up with an
electric wire wheel, and painted it. It looks fabulous, and shouldn't crack
again, thanks to the reinforcement.