I came close to building a 404 maybe 15 or so years ago when it seemed to have been resurrected with a number of folks building rifles on AR. I held out, moved a number of times, and once settled put together a 416 Remington after owning a Dakota 416 Rigby in the moves. The Remington round just made sense to me and my tweaked New Haven M70 has proven itself well.
416 REM is an easier one to build than .404J.
DF