I may have answered my own question - a look at the substitute coils listed for the 380SL shows that the coil's primary resistances are listed as being from 0.6 to 0.9 ohms, and the 280Z coil's primary resistance is also in that range. So I think they should be swappable. I have another ignition control unit on hand, a hueco "functionally equivalent" unit, so I think I should be ok for my 400 mile trip tomorrow.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C