I used muffler cement (Quaco brand) on an exhaust manifold, right on top where it came out of the head.  I cleaned it well and it lasted the rest of the time I had the car, 50-60,000 miles.
I think muffler cement works better on cast iron that a muffler that is expanding with exahust temps more than a cast iron manifold.  You don't have much to lose trying!  
 
John