I got a reply from MercMad63 and thought to share it.

First my e-mail to him:

I am from South Africa and new to this forum and have recently acquired my
first old Mercedes Benz. It is a 1968 W108 280SE body that the previous
owner fitted with a 1982 380SE V8 engine and auto transmission. The original
differential is still in the car and the final drive ratio needs to be
changed. The original diff's ratio is 3.92 - I think. I think that the final
drive ratio of the 380SE M116 engine is 2.47. So - obviously not a good
match!

I have been advised to try and find replacement axles from a W108 4.5 or a
W109 6.3 lt. But here in South Africa they are like hens teeth....

What would you advise me to do?
(a) Send the diff to an engineering outfit and have the final drive ratio
changed to 2.47? (Quite costly.)
(b) I suppose an axle / diff from a later model 380SE would not fit the W108
body?

Or what other solutions?

His reply:

Hi, I have done this xswap a few times . Yes you need to up the final drive.
Your other option is to buy a diff from a W108 280SE 3.5 whyich 3.25:1 and
bolts in or import the crown wheel and pinion from the USA for the 4.5.
If tyou want tyo save a bit of money swap in the whole rear end from a W126
(380SE) . you will need to weld in a brace for the diffs rear mount. Weld in
a bracket for the two front mounts and get the springs remade because the
originals are too soft and the W126 springs will not fit. It sounds like a
lot odf work but it's actually quite simple.

My questions:

@ MercMad63: Have you got some photos of the modifications of fitting the W126 axle on a W108 chassy?

@ Every one else: Do you know of a crown wheel and pinion for sale as suggested above? What other parts do I need to replace when changing the crown wheel and pinion in the W108 diff?

Thanks in advance!