Not having a milling machine, I spent approx 10 hours drilling, filing and polishing, which also included throwing out the first attempt as I did not like the way it was turning out. The filing was the killer.
Drilling the pilot hole
Lining up for drilling the Zenith mounting holes
Anyone for Swiss cheese (or death by a 1000 holes)
Many hours later, the finished plate
And a few shots of how it all fits together
You may notice a gap between the manifold and the adaptor plate on the following shots. There is a bit of interference between the edges of the manifold flange and the bolt heads securing the Zenith to the adaptor plate. This will be a simple fix requiring slightly reducing the edge of the manifold flange and possibly fitting socket head bolts instead of hex head.