If you are wanting to shorten a standard bbl you will have to move the gas block back and use a shorter gas tube and piston. Do a search, there was somebody doing the shortening process on the whole front end for people. While you *can* do it all by hand or with a bandsaw, a lathe makes it much easier.
Building from a flat, no trunnion holes is usually best. Every country located their holes slightly differently, some may even differ between examples of the same model. No holes lets you drill them precisely where they should be instead of possibly having to egg shape them to line up with your trunnions. If you are bending from a flat, the inside measurement should be the same regardless of thickness, you are the one bending them so you decide what the ID will be.