Sorry, you can't make a case head bigger. If you want to build something like a 30-378, etc, you are going to have to use Weatherby brass. Other options are those based on the 338 lapua, and 416 Rigby case. You can also consider proprietory cases like those from Lazzaroni.
Refer to posts by Darryl Cassel on his 338-416 rigby improved monster.