I've been told to use manifolds that aint too old or too used. Or else they dont have enough "carbon" left in them for the weld to stick. I also remember them saying to always use a nickle rod. The cast iron of an exhaust manifold is simple to weld compared to the cast iron of an engine block/head or transmission case. You will find out pretty quick if the manifold has enough "carbon" left in it or not if all you can do is make holes.
All of my splits were done by a fellow who preferred to go into the heat riser area and weld in a "blocking plate" within the manifold. The lack of an operable heat riser flap in Texas never was a problem, and it left the manifold in one piece making it easier to install. My now deceased welding friend could even use plain exhaust head pipe with the 2-bolt flange as a new outlet and weld it to the cast iron manifold.
