Are the W140s and the R129s interchangeable?

Absolutely not! Although they appear to be identical, and they will fit in each respective car, the operation would be incorrect. Here are the differences :

W140 gas tanks are 100 L, R129 are 80 L. The fuel gauge reading would be incorrect.
R129 transmissions have gearing before the VSS, which means that 320, 500, and 600 clusters are designed specifically for their application. All W140 clusters are speedometer compatible.
R129 VSS and tach signals have negative voltage biases, which cause misreading from W140 clusters.
As a result of the uniqueness of the R129 cluster, it is a more difficult unit to find second hand.
http://mbcluster.com/