I think that the only rear gears that would fit the 300E and give you a higher ratio (lower numerically) are the gears from a diesel 124 (or a 300TE).
If I remember correctly, cars with lower than about 200 HP had the 185mm ring gear in the 1.1 liter fluid capacity pumpkin. Higher than 200 HP cars had the larger 210mm/1.3 liter setup. So you would need the smaller size setup, because I don't believe that they interchangable.
Your 300E gears are probably 3.07:1. The 300TE and E300D had 2.87s and the early 300D sedans had 2.65s. I think those would be your only choices. (although possibly another model's (like a 190) gears may fit? ...beyond my knowledge)
Note: your 300CE has the larger read end with a 2.65 ratio.
I'd suggest you drive your coupe.
