Hi Moxis,
You need to start with the flat faces. Yep they are a bit thin and the flange looks impossible to hold and machine without causing damage - it is difficult so we are not going to use it
Give the casting a fettle get rid of any lumps on the front and back faces. Give the gasket face a rub on a bit of abrasive paper to get the surface reasonably flat. Measure - check you can get the flange thickness out of it and where the excess metal is. Decide how much you can take off the back and how much you can take off the front.
Find a bit of scrap bigger than the front flange and put it in the chuck and face it. Using either a drill chuck (or in this case I used an ER 32 chuck as the flange was too small to grip) hold the casting by the boss in the tailstock and super glue it to the scrap in the chuck. Let the super glue set then turn the flat, the boss and drill the centre.
Turn it over and hold by the boss. You options are to use a collet chuck like a 5C which can hold short bits or to make a pot chuck which is a tight fit on the boss. In the worst case you could just counter bore the bit of scrap for a good fit on the boss and then superglue on again to face the back
)
Now you need to find where the mounting holes are to go. I hold the casting by the flange in a collet chuck that I have centred and drill the holes. In the old days people would make a drilling jig that slipped over the boss and guided the drill to where the two holes need to be drilled - just make sure you find the centre line that gives you maximum area for where the nuts are going to go. I would check the nut/boss spacing first as you can see from these flanges there was no space for the nuts to rotate and I was forced to use smaller studs/nuts
As Jason said the easiest way to shape the outside is using filing buttons
Jo