Yeah, I think they all have a shallow draft angle to help get them out of the dies. When I had access to a mill, I used to just clamp them lightly but securely between the front bearing and the back plate surface with a soft block or aluminum plate. Then when you do the cutting, take it just a few thou at a time going from front to back, it doesn't take much. If you have a good set of files, and a steady hand and good eye, you can file it with out much effort. Just have a good mounting surface the right size to check how the engine sits on it. Most are not that far off, but the occasional one has some sort of error in it and when you tighten it down on a mount it flexes the case. If you throw in a mount that is not level and parallel, you are in bigger trouble. People were not aware of this back in the day. That is one reason why so many f the old engines you find have so little time on them and need breaking in. Not everyone was successful at this hobby back then and failed to stick with it or seek help, and no internet back then. If I ever get my garage squared away, I want to add a small table top mill just for proposes like these and for repairing welded up welded mounts.
Type at you later,
Dan McEntee