Take a look at this link and see if it provides some orientation - car in the pics is a 84 300sd
http://www.peachparts.com/shopforum/showthread.php?t=206187&highlight=oil+drain
The two piece tube you show in the first two pics looks like the drain that should be coming from the bottom of the turbo.
What you want to find is another opening just to the right of that turbo oil drain tube (moving inboard going closer to the block) where the drain that normally comes from the bottom of the stock air cleaner assembly snakes down and via a metal tube and enters into the oil pan (about 1" or less from the turbo oil drain tube). The smaller diameter opening is where the check valve is on the write up link above. The pics in the link are from the bottom with the oil pan off looking up. On the opposite side looking down past the turbo you should see a little brass nipple that is on the top of the oil pan that normally has one o-ring - that is where the tube from the botton of the air cleaner enters the oil pan.