My mechanic says he has found an early model M104 short block soon to be for sale which should fit the M103 head. I told him I was pretty sure the M103/M104 ends were not interchangeable but he says this particular block should work. He's going to call the lead MB technician in Australia on Monday to get confirmation.
I don't want to get my hopes up as this question has come up numerous times and each time the verdict has been that under no circumstances are the M103/M104 ends interchangeable.
My mechanic told me that there's not a single M104 for sale anywhere in the country at the moment (the one he found is still in the wrecked car and won't be removed for a few weeks yet).

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1989 300CE "Project HWA124"
(400rwhp Turbo Technics AMG C36 engine)