My two bits is that the old method can still work. I've hade a homemade split manifold on my 230 for 30 (yes, thirty) years with no problems. With nickel rod and some patience this can be done. I picked a spot on the side of the ex. manifold behind the center outlet. Drilled as many 1/4" holes in a circle as it took to remove a round section of the manifold wall. Cleaned up the edges with a grinder, and welded on a water pipe elbow - 2" I think. Cut the bottom flange off a scrap manifold, and welded that to the bottom of the elbow.
Then ahead of your new outlet saw thru 3 sides of the manifold in 2 places just far enough apart to insert a section of 3/16" mild steel plate as a block off. Then weld around the 3 sides. There will be some leakage between the front section and the back, past the 4th side of the blockoff, since it is just touching the back wall. But this will act to cut down a bit on the harsh rap rap produced by a complete split. If you can, have the manifold bolted to a spare head when you do the welding, to minimize warping. After you are done, take it to a machine shop to have the block mating surface planed.
