Length isn't a big issue, I've used ½", 5/8" and ¾" long screws. The oldest barrel that I have is 1991, it has the coarse thread pitch, I think only older pre-'87s used the fine threaded studs, both are listed in the FAQs for anyone that cares to look, there's even an alternate metric size to use in lieu of the ¼-28 screw compliments of MSP.
Tim