why dont you get a piece off inch plate,drill wholes in it to match the manifold bolt holes and clamp it down with a 6mm packer in the middle of the manifold flange,then heat it up in the middle and slowly tighten the end bolts down until you have taken the 3mm of warping out off it,just a idea if you carnt find anyone to machine it for you,it might sound a bit of a bodge but it does work.