This block vent / drain fitting plate from Yoshifab has been designed to replace the original Volvo breather box under the intake manifold. It has two ports. The small drain port on the left side has a -8 AN Male fitting. The larger vent port on the right side has a -10 AN Male fitting.  It's needed if you plan to plumb a drain from a catch can to this drain port.  If you will not be draining oil here, this fitting is not required. If you're using 5/8 inch ID hose, that size fits perfectly on the vent port on a factory Volvo oil trap box.

In case you're not aware, the Volvo B21/23/230 engine block has a long plastic tube inside that extends from that drain port above down into the oil sump.  The bottom of that tube is submerged below the oil level. When this drain is extended below the oil level, it ensures that this pipe drains freely and does not get any interference from positive crankcase pressures.  So basically this Yoshifab plate is designed so that when an external catch can is installed, the bottom of the can SHOULD be placed somewhere reasonably close to and ABOVE the level of this drain port.  This is not always convenient in a Volvo due to tight space, although there seems to be plenty of people who have managed to accomplish this.  I did not as you'll see a bit further below. 

As seen above, the oil drain below the stock crankcase vent box for a B21/23 or B230 extends down past the oil pump and ends just above the bottom of the oil pan.  It's designed to end BELOW THE OIL LEVEL, so it doesn't get affected by elevated crankcase pressures due to boost.

As you'll see in MY installation below, I opted to NOT use the drain port on this block vent fitting. 

That drain port can be capped off with a -8 AN Female Cap, such as this item from Summit Racing: Earl's  -8 AN Cap (AT992908ERL).  This is what I used, since the drain I used was on the other side of the engine.

Here's my 240.
Things are harder to see in this photo. I decided to mount the catch can on the right (EXHAUST) side of the engine bay, in the back corner.  My drain from the can down to the crankcase is on the exhaust side. I'll explain more below.

A HOT Catch Can:
One thing about putting the catch can on the EXHAUST side . . . It gets hot.  Is this bad?  No, I don't think so. I actually think a HOT catch can is much better.
Some high end catch cans are available with heating devices, such as ports that run hot coolant through them.  A hot catch can will have the potential benefit of turning any moisture into vapor faster, which will hopefully then slowly exit out with other vapors and not stay in the can or return to your sump. The hotter the better in my opinion.

DRAIN ROUTING TO THE SUMP
My plan routed the catch can drain down into a Y-fitting that joins with the turbo oil drain just above the block drain.  This Y-fitting tends to make things a little complicated down there as you can see here, so some planning was needed for all these AN hose fittings to fit in a small space.  The turbo oil drain that I previously installed uses Summit PTFE lined racing hose, which is a high temperature hose designed for hot oil (rated to 400 degrees Fahrenheit). So I used the same type PTFE hose for the catch can drain. I wanted to use a hose that was rated for high temperature since this stuff is so close to the exhaust manifold and turbo.
 
  I would prefer to NOT use this Y-fitting. I would rather have a dedicated drain port down low on the side of the oil pan (below the oil level), but this is something I think I'll work on later, since I didn't feel like removing my oil pan at this time to drill a hole. More info on THIS: CLICK HERE

Here are the connectors used to assemble this Y-fitting.  The Y-fitting has -10 AN male ends. It uses a reducer to -8 AN for the hose that goes up to the breather box.

Not shown here is the turbo oil drain (seen in above photos), which connects to the other upper end of the Y-fitting. That hose uses a short piece of Summit Racing -10 AN PTFE hose (SUM-2201010), one Summit Racing PTFE Hose End -10 AN 90 degrees (SUM-250087B), and one Summit Racing PTFE Hose End -10 Straight (SUM 250090B).  Specific detailed information about the turbo drain hose fitting that fits into the block can be found in a separate article HERE.

Assembling PTFE hose ends is not difficult, but you'll need some guidance if it's your first time.  There are good instructions here: https://anfittingguide.com/install-ptfe-hose-fittings/. 

And a good video below.

OIL DRAIN CHECK VALVE
I installed a one-way check valve in the oil drain hose between the catch can and the Y-fitting. It needs to be placed so flow goes down only.  A check valve is used in this situation because the drain goes into the oil sump above the oil level.  A different option would be if the drain went into the sump below the oil level. 
A check valve WOULD NOT be needed if this tube drained to a port BELOW the oil level.

You can't see it in this photo here, but I used a 45 degree adapter (-8 AN Female to -8 AN Female) between the catch can and the check valve.
Like this one:  https://www.jegs.com/i/Earls/361/AT939208/10002/-1

More detail about the drain check valve is below.

 

This is the check valve. It accepts -8 AN fittings on both ends and can be found at Summit Racing here: One-Way Check Valve -8 AN Male (SUM-220193B). 
The summit instructions will tell you not to screw with the spring that holds the check-valve closed.  I couldn't stop myself.  I felt it was too tight, so I tweaked it until it was much easier to push open.  I want this to drain freely and not back up into the catch can.  I'm pretty confident in my decision.

Here's a pic showing the 5/8 inch clear PVC tubing in use at the catch can. 
That top port got routed through the back of the right strut tower, then to the front of the engine bay. It's connected to the intake air tube between my air filter and turbo. That is a 150 degree curved fitting I used on top of the catch can. This one: -10 AN Summit Racing Twist-Tite Swivel Hose End 150 Degree (SUM-260091B).