Rhd issues aside, what about making it a step-up style and starting off with the factory manifold, cutting 6" away from the flange and then tig larger pipe and so on. Just a thought, my Datsun headers started off at 1.5" then 1 5/" then 1 3/4" into a merge collector. Long-tube benefits with supposedly no loss in top end. Plus the flange and first bend would already be done for you. I would tig around the outside of the pipe at the flange so that you could grind the weld smooth on the inside. Just a thought, I've never designed a header.