Correct. The 603.961 is in the 300 SDL from 1986-1987. The 603.970 is in the 300 SD and SDL from 1990-1991.

Now you are making the assumptions.

The difference in engine size between the two is 17%. I don't have the specs on the 3.5L, however, I seem to recall it being a lower speed engine. Therefore, I conclude (by sheer speculation) that the stroke is increased from 3.31" to 3.87". The bore remains the same at 3.43.

If this is true, the theory of the head gasket letting go earlier on the 603.970 engine would not be applicable. The head gasket suffers from the same stresses as it's older sibling. The rods, however, are travelling a bit faster due to the longer stroke and are therefore more highly stressed in the larger engine.

Of course, you can conclude that M/B designed additional strength into the rods for this express purpose.............................