Did some more work today.
Using my pillar drill and an end mill, removed the lump around the original centre stud.
Using a bakelite manifold gasket, I made a template for the two lower studs and drilled the end castings.
Endcasting and runners fitted. The endcastings are alloy but the runners are steel. I need to cut off the runners so I can weld on an flange for the throttle bodies. I haven't decided on the height yet, I need to fit injectors and rails before I decide on the whether to make the throttle body flange out of aluminium or steel.
As the runners curve almost immediately, I think they may end up being aluminium. To remove the runners and get the height the same I will hacksaw off the runners, and then bolt two spare heads together (so that they are sitting horizontally) and have the runners milled off.