It has always been legal to manufacture your own guns IF they are for your personal use and not to be sold. Manufacturing for selling requires a class II FFL.
In this particular case, I believe the defendent had true machining knowledge and basically milled his own receiver. I'm sure this will never be extended to just making your own DIAS or lightning link.
For anyone looking at Garands, please look to the CMP first--one of the few things done right in this country regarding guns.
http://www.odcmp.com/