I did almost the same thingto split a stock manifold. One thing youmust remember when welding cast is to either preheat the whole manifold, weld, then bury it in a pile of sand so it cools slowly or only weld about
3/8" long bead at a time. Wait til the next day and weld another 3/8" and repeat that til it is all done. If you introduce a bunch of welding heat in one spot you will probably crack the manifold. Good luck
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Gael
37 chevy sedan, 261,t5
57 pickup
58 burb
