Joe, I can try to help. You can still move the whole barrel assembly back. Go slow and check often. One way to make sure you get it right is to scribe a center line on the flat that your vent will go in. That line should run horizontal down the length of the flat. Next, scribe a vertical line where your breach plug face ends on that same flat. Then measure the diameter of your vent liner, divide that by half and add about 1/16" to that half. Using that measurement, scribe another vertical line that distance forward from the breach plug face line. Now, where the last vertical line and the center line meet, use a punch and put a little mark at their intersection. That will be the center of where you will later drill for you vent liner. As you know, you want the liner hole centered on your pan in the sun set position.
From the picture it also looks like you need to go deeper with your barrel channel, which will let you get your vent in the proper place - vertically. It will also allow you to take some of the sharp bend out of your tang and flow into the wrist area.
There are other ways to accomplish the task and others may chime in.