[rbrown1]

You are getting close to the max size for investment casting process. This appears to be a cast iron type part. As for how to model, typically all but the finished machined areas are cast to size. The area's that will be machined need to have approximately .100 material added to the model to be machined after casting. For a sand casting without the fabrication of special cores (requiring more tooling and increasing the cost) the walls need to allow for the pulling out of the tooling very similar to an injection molding process. The tooling for sand casting last for long time so you can cast 1 part or hundreds. I hope this helps if you need more information either post here or pm my account.