Thought the first year of the electric speedometer was 1985 123s.. Glad to be wrong. Maybe I was thinking of the 126 series?

I was pretty sure all prior to 1985 123s where mechanical anyways. Any 123 turbo with a 307 will be mechanical I suspect. So you should be good to go.

My 1977 300d with a 3.47 sounds a little busy on the highway but the two 616 models with their 3.69s are worse.