When installing the barrel extension, is there a washer between the extension and the barrel or does it get tightened to align the hole at top-dead-center?  If so, what type of tool could I use to tighten the 1/4 turn I need to align it?

There is no washer or spacer between the extension and the machined shoulder on the barrel itself. It is a precision made assembly and because of the feed ramps and the location of the barrel pin, it requires a lot of detail. It is torqued onto the barrel with 175 ft. lbs. of torque.

A special tool is required but some folks have claimed success at removal / install using an old bolt. The tool used to be available from Sarco but there may be other sources I'm not aware of like Brownell's.

The gas port hole and front sight post pin holes are drilled AFTER the barrel extension is installed.  Replacing it is next to impossible since the extension lugs will probably not align with the sight / gasport.