Wow, what a beast..! For what I heard, the 5 speed gearbox is actually stronger than the 6 speed gearbox..
Perhaps your solution be an inverted G50 Porsche transmission that is designed to handle way more torque than the gearbox you are using, but those gearbox are pricey..
You may want to ask CMS as they have a lot of experience with these gearboxes
https://californiamotorsports.net/
Good luck, and please share more pictures of your Boxster (..?)