KCFirepower,
The head spacing and barrel fit are critical for a FAL to be safe and work correctly, it takes specialized gauges and tools. You should be able to handle most all of the rest of the build. I trust a professional for the upper work, and not just any smith, one that knows FALs. The locking shoulder will usually be provided by the smith that does the work.
You might look into this fellow, I have heard good things about his work.
http://www.cgwgun.com/FAL-Turn-Key-Upper-Assembly.aspx
This guy has done a pair of my guns, but he is hard to get a hold of and his work takes a while, it's good, but only if you are not in much of a hurry.
http://www.vandenbergcustom.com/Content-Public/Rifles/FALs/Page.html
Also remember, just because some smith worked on the part does not make it compliant. If you keep your internals, you will have to get compliant somewhere else in the build. Buy the new American parts, then get them tricked out if you don't like the way they feel.
Here is a photo of my FALs,
(Waiting on the W.E.G. magazine foul comment, LOL Other than being a funny picture guy you know he is an admin over at the FALFiles, right?? A good source of information if he chooses to jump in.)