Pentoman:

I seem to recall seeing on at least one U. S. based radiator supply web site, the 260 and 300 using the same radiator part number. I may stand corrected. You might try surfing the web using something like EXCITE, looking for radiator suppliers. Look up what they offer for both the 260 & 300. It might be interesting to learn they are interchangeable.

Not sure about the other areas you inquired about; however, according to the specs. found in Robert Nitske's book "Mercedes-Benz Production Models Book 1946-1995", the biggest difference between the 2.6 and 3.0 motor is the bore. Stroke is the same. I wonder if engine height is identical? I believe there's a fair amount of room between the very top of the air cleaner assembly and the hood. It might just fit!

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"