I'm in a similar position. I bought a 1976 300D. I changed the glow plugs - and the fuel filters - and it wouldn't start any more.

Is the relay in our model different than the relays in the 300D W123 model? I notice that other parts of the system are quite different. If it is the same relay as in other 5 cylinder diesel models, and you are changing it just to see if changing the relay will fix the problem, it might be easy to find.