If it were me...
I would turn a piece of 8620 steel down to where it would just slip-fit (just a slight-interference fit actually) into the hole that is out of position.
Then I would cut the plug .250" shorter than the width if the trunion so that the ends of the plug sit beneath the surface of the trunion.
That would leave some room for me to TIG-weld the plug permanently into place using some 80-NI filler rod.
Then I would (just snug) chuck the trunion into my cnc-mill and dial-indicate the trunion flat, tighten it down, and mill the welds flat.
I would then take the receiver and chuck it up into the mill.
Edge-find the top, and the front of the receiver and set that as my "zero" (i.e. origin) coordinate.
Set up either a wiggler or a center-finder in my quill and establish the coordinate positions of all three holes a, b, and c and record the positons into the cnc-controller.
Then I would take my trunion and chuck it back up into the mill and edge-find the front and the top, just like i did the receiver.
I could then check the positions of holes b and c by running the mill over to those and checking with a reemer or a gauge-pin.
If holes b and c check-out, then I would run the mill over the the previously-recorded position for hole a and drill/ream the new hole and be done with it.
Should probably take 2-4 hours of screwing around to complete.