303mm discs doesn't quite sound right.  I'm currently setting my 300TD up to
autocross and have acquired 334mm fronts, with 300mm rears from an SL600.
I've attached a link to a spreadsheet with tons of data about MB brakes,
though I don't believe it covers any info about disc offset.

http://www.w124performance.com/images/W124_brakes/124_brakes.xls

Casey
'89 Bluestar
'94 100CSQ Avant
'87 300TD modified