The first thing I do is determine where the bottom of the FH will be. If you have a Phantom that needs to be indexed (closed bottom), it is easy to determine the bottom, but you you have the fully open type, test fit the Phantom to the barrel and find and mark where the bottom is.
Next measure the thickness of the Phantom at the marked bottom. This will determine the length of the roll pin you need to pin the FH. Add 1/16-1/8" to your measured length to compensate for the amount the pin must engage the barrel.
Next, drill the proper sized hole at the marked bottom of the FH. Install the FH and tighten to whatever spec you prefer. Then you need to drill the barrel 1/16-1/8" deep into the threads. I use the same drill bit used to drill the FH, insert it into the hole of the FH and mark it with tape 1/16-1/8" higher than flush with the FH. Drill the barrel until the tape touches the FH. Drive in the roll pin and clean it up flush if it isn't already there.
Now it's ready for welding. I use TIG. I strike my arc directly over the hole and get the pin and surrounding metal molten and add a little filler rod and it comes out nearly flush with minimal cleanup required. Repark or recoat with whatever you like to use.
I've also done this with setscrews using the same basic method described above. The following poor pic is a Vortex set in place with a cone tipped setscrew prior to final weld.