Well, there is a small issue here, your MB dealer provides you with white batteries these days, but when these cars were new the batteries where black, and
they where delivered from Bosch.
http://www.bannerbatterien.com/banner/home/index_en.php deliver batteries with the right measurements and
if you tear off the Banner stickers and replace them with a Bosch sticker, witch you will find on ebay i would suppose you are up to the "originalgefühl"..
With the Banner battery mounted you will also be able to use the original mounting top frame. The 3.5 has the battery mounted on the other side of the
engine bay, and it is also bigger than the one for the straight six cars. But also the correct 3.5 battery can be delivered from Banner..
Here is the engine bay of my 280SE/Ca with this battery mounted: