So I might have an opportunity to get non-EGR manifolds and a full non-oil separator air cleaner/breather setup from a Euro 617 turbo. It's the same setup used on the non-EGR W116 300SD. I removed the EGR on my 81 300SD and put a blocking plate there, but I'm tempted to make it look even cleaner. The issue is how will I plug the drain at the upper oil pan if there's no oil separator anymore? I wouldn't want it sitting open to the elements even though there's a check valve in there.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

In your case I would recommend that you not plug the drain.
Continue to use it for its intended purpose.
Remove the cyclonic oil separator from your old air box (or use a catch can) and plumb it to the drain in the upper oil pan.

The 78 & 79 W116 300SD do have a rudimentary crankcase vapor recovery system,
that is not as efficient as the cyclonic oil separator.

There is no feed of collected oil to the sump.
There is a "tee" fitting on the cranckase vapor recovery tube.
The relatively oil free vapors exit the tube horizontally into the base of the air cleaner box and soak the air filter as well as form a puddle in the air box.
The coalesced oil droplets (that don't exit into the air box) flow down the "tee" into the air intake hose to the turbo...and also leak out onto the alternator and the PS motor mount.
It creates quite a mess, and is not an ideal design in many ways.
Feeding oil into the fast spinning blades of a turbo might shorten its useful life.

Presently I'm fabbing a bracket to mount a baffled catch can on ''Desert Rose' and will submit a post with photos once completed.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

I guess in that case I'll keep the original air box with the oil separator and just install the non-EGR manifolds if I can get them.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

That would be the simplest solution, I didn't think your senario all the way through.

If your source for non-EGR manifolds does't pan out, send me a PM as I have a spare set in storage and could use the funds at this time.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022