The spare engine I have is an early 123 so it has the same ac mounting and oil filter. I was just out looking at it more closely. It looks like the thermostat housing is slightly different to accommodate a vacuum sensor on the newer one. There is also a tach sensor above the flywheel on the newer one which must be for the cruise control. I wonder if there is a magnet in the flywheel of the older one. I have never had a 240 with cruise control and am looking forward to having that work.
The throttle linkage is very different because the newer one has an egr and an oil separator. The newer one has a drain port in the top of the upper oil pan to accommodate the oil separator so I can't swap intake manifolds to make it work.
I actually have an 84 grey market 240 to look at for reference, but it is way different from both the 83 and the 78 with yet a third type of intake manifold, and no remote oil cooler.
I just took the quick coupler apart on my compression tester and found the bad oring that was causing it not to work. I may do a compression test on the 78 engine this morning to give me more data.

__________________
1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660