Baron,
The bracket would have been available also from the MB dealer ... but anyway.
Since you are in the middle of your restoration process, fuel in the lines is probably not an issue for you. Otherwise I recommend to take the lines off during welding - at least under the front floorpan.
Riveting is not the choice here, at least it is not original but welding is. This is what I would do as well in your case. Otherwise - if you are scary about the exact location - you can use screws. This is easier to readjust if necessary.
Please keep us updated on the process.
Best regards,
Achim