Splitting a stock manifold isn't impossible, but the welder definitely has to know what he's doing. It isn't necessary, or desirable to completely block off the halves of the manifold- - - -leaving a small gap to allow a pressure relief is best. With a thin cutoff wheel on a small angle grinder, a gap about 1/4" wide needs to be created in three sides of the manifold just ahead of the heat riser, and a piece of cast iron cut from a junk manifold is inserted, leaving maybe a 1/8" gap inside at the top where the manifold doesn't get cut. Grind the area to be welded down as smooth as possible for about 1/2" on both sides of the baffle and chamfer the edges a little. Now, use nickel-alloy welding rod to make about 1/2"-3/4" of weld bead, after preheating the manifold with a rosebud torch until it's almost, but not quite red. Stop welding and peen the bead for about 5 minutes with the pointy end of a slag hammer to stress-relieve the weld. Heat- - - -weld- - - -peen- - - -"rinse and repeat" until the whole baffle is welded into place. Post-heat the manifold, not quite as hot as the preheat and bury it in a pile of sand overnight to cool slowly. It's also a good idea to have the manifold bolted to a junk cylinder head, or at least a piece of thick steel plate to prevent warping while you're welding.
Once the baffle is welded in (and the weld bead didn't crack) use a hole saw to make a hole in the front of the manifold and weld another pipe flange on, using the same procedure you did to install the baffle. Have fun!
I found a roll of nickel-alloy wire for my MIG welder, so the next time I have an occasion to weld cast iron, I'm going to experiment a little to see if I can develop a procedure for using MIG instead of stick welding.
Jerry
