I don't much care for railed gas blocks or anything fancy. Just a standard A2 FSB is what I like.

I have a disappaitor barrel laying around that is stripped, but is already drilled for an FSB (must have been removed at some point).

I've thought about getting the Armalite clamp on A2 base (like here) or Fulton Armory's which kinda the same.

The alternative would be getting a new FSB and drilling it myself. I have a big ass drill press so drilling isn't the problem, But how big of a problem is it lining things up right on the new FSB with the existing holes?

It can certainly be done but alignment prior to drilling and taper reaming is a challenge.  I would just recommend a clamp-on or set screwed FSB.  Much easier to get final alignment if regularly using iron sights.