u could drill each end of the crack witha small drill, grind out the back of the crack - get the manifold really hot (like in an oven) and weld it all back up using a cast iron rod (they do exist) and then back in the oven and let it cool down slowly of its own accord (with the oven switch off) . this should stabilise any expension/contraction causing the weld to crack prematurely.

some more info here : http://www.twi.co.uk/j32k/protected/band_3/jk25.html