...Hmmmm...No armorers here...Or they don't have digi-cameras...(or not talking)...
OK; here's my current thought: Using a 1/8" drill rod to align the stripped bbl in a bbl vise, in a machinists vise, in the mill. Remove the rod, replace with center drill. Slide flip-up FS/gas block and handguard cap onto bbl. Use machinists level to level gas block/FS. Clamp gas block lengthwise to bbl vise. Center drill the gas block, followed by 1/8" drill through to bbl hole.
At this point I'm undecided whether to try drilling straight through other side, or flip the whole thing over & repeat. It takes longer; but that's probably what I'll do...
I've replaced FS/gas blocks before, and have found that a # 3/0 taper reamer is not enough to get a good smooth taper (production holes I've seen are really rough); I usually finish up with a # 2/0 reamer and longer pins (milled in place to proper length).
Just thinking out loud...