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.