I would do it on a milling machine. Dismount the barrel, insert a mandrel and set them up on a milling machine. Touch off on both sides the mandrel to find the centerline (you want it so it is aligned/parallel with the bore).
Here's the part I can't do w/out a schematic: Where do the holes for the pins go? If the drawing is from the face of the receiver, then it is matter of moving the table so the mill will be over the spot to be drilled. After the first hole is milled out, then move the table the distance required for second pin hole. A DRO helps here. After the two pin holes are drilled, dry run assemble. If everything lines up, tin both the receiver and the mount. Drive in the pins and heat the mount and receiver up to with a propane torch solder them together.
The factories had it easier. They made jigs so the worker didn't have to fiddle faddle with adjusting the workpiece on the mill. Mount the jig on the receiver and put them on the milling machine and off you go!
Hint: take it to a gunsmith with a milling machine. Even then you will have to provide plans to show the location of the pin holes.