Best I can tell from literature, the 300E 5 speed ratios are:

I 3.86
II 2.18
III 1.38
IV 1.00
V .8

As long legged as this car is geared, I believe those numbers. The '86 and '88 300E's had different manual transmissions, so this might be wrong for my '88. This car would really benefit from a 3.69 rear gear IMHO.

Also, if you are selecting a trans for a performance application, the one from the 2.3 16V would be stronger because it is straight through in top gear. The ratios would also be better with a tall rear gear.

Have a great day,