I have a C36 engine swapped into a w124 wagon. I have the parts needed to go the getrag swap. I am planning on using the stock 2.65 rear end as the getrag does not have an overdrive and therefore would be the same on the highway with the 4 speed auto.

If you use the 300e overdrive unit a 3.07 rear end would be a nice option. Here is a good chart for all the w124 gear ratios:

http://www.w124performance.com/docs/mb/other/differentials.pdf