I drilled the FH
screwed it on
drilled though the threads of barrel (only) with a 1/8" bit, using the FH as a guide.
Flipped the drill bit around in the drill and touched it to a grinder, giving it the same angle at the cutting end.
stuck the non cutting end into the hole and marked it, pulled it out and cut it off.
Stuck it back in, trim to final length, and weld it.
Do everything but the welding, and that would make it easy for him.
If the pin goes in as deep as the root diameter of the threads on the barrel, its not going anywhere without shearing the pin and destroying the threads to get off.
If you drill into the barrel threads, tell you have no threads showing, your slightly deeper anyway because of the point of the drill. This is why I ground the bit to match the cutting angle.
It may have taken me longer to type this out with my 2 fingers than to do it.