Precision Firearms or GAP, though they might be too slammed to do it, but that’s where I’d go first.
I’ve had it done before with a short-chambering. Bill A. actually did the work on my Lilja, which is why it’s a 17.6” and not an 18”. That’s my 18”-ish workhorse Grendel. That barrel is light but still shoots and holds rapid strings on-target.
A good machinist knows how to time an extension to match the previous clocking so the gas port stays aligned TDC to 12 o’clock. They have formulae for torque, thread depth, pitch, etc. and the angular rotation needed to land on correct clock position.
I have another source but he is extremely selective on what work he takes in. He’s an artist with machines. His thread jobs look like a CNC did it with polish afterwards. Thread fit is perfect wit no slop at all. Other senior gunsmiths have called him a liar when he told them he did his thread work by hand on his lathe.