KitchNZ, To get my inlet tube from round to oval I was going to machine up a piece of steel the same size of the ports i'm using, weld it to a piece of flat steel, heat up the inlet tube and swage it over the port size piece of steel by press and then manipulate it to shape, slicing and rewelding the inlet tube on the under side may be needed.
You only need to do this for about 20 - 25mm of the inlet tube, enough to make the shape for welding.