In order to use the standard 0.25"NPT shop air fittings, use a 7/16" drill bit. Drill one hole on the end to a depth of 0.6". Drill the other end to a depth of 2" using the 0.75" end mill. Be sure to use plenty of lube & back out regularly to clear chips. Go back to the end where you drilled the 1st hole & drill a 0.2" hole in the center of the 1st. Drill all the way through to the hole made by the 0.75" end mill.
Now, on the top surface, start at the end of the 0.75" hole & measure back 1.78". This is the center point for your next hole. Drill this one with the 7/16" bit. Stop once you clear the wall of the 0.75" hole. do not drill into the other side of that wall.
Next step is to tap the holes. Put the block into a vice & tighten securely. Take your 1/4NTP tap & carefully start cutting into the hole. It's best to use lube & back out regularly to clear out chips. Do the same on the other hole. Now your ready to install the quick disconnect fittings. Make sure to use teflon tape.
Once you reach this point you can test the rig. Hook the horizontal QD fitting to your shop air. when you apply sufficient air pressure, it will generate a vacuum in the vertical fitting. You can test the performance of this setup by connecting a vacuum gauge to the vertical fitting.