These guns are difficult to build. I have a parts kit sitting in my spare room right now as I ordered the kits when they were cheap ($49 plus shipping). I lack welding capability and don't have the extent of machining ability that I thought I would. So, mine has remained unfinished while I work on another project.
There are several hurdles to overcome: Creating a new receiver, modifying the FCG for semi-auto only fire, and (perhaps most difficult) machining the bolt to fire when closed instead of from an open-bolt configuration.
I was planning to build mine as an SBR, so I could only point you to the original length barrel. I have seen 16" barrels out there. I would point you towards Prexis, as well as Greg Clark from gunbuilds, as potential sources for more information.