Sounds like your "moonlighting" friend needs to pay for that vacation. It doesn't matter whether you have an early or late cam, none of them can have their replacement needs diagnosed without visual inspection.
The original 300D cams placed too high of a loading on the rocker arms and they failed early (as early or earlier than 100k mi). They redesigned the rocker arms and maybe the surface hardness of the cams at some point (might be 1977 but I know there was a design change in 1980 also), I think all SDs had the new rocker arms so that would make it by 1978.
Bosch Master, ASE Master, L1
33 years MB technician