Your smith is FOS. I've threaded 3 SAR-1s. As long as your barrel is .550+ you're all set. Also your FSB already has a spring loaded detent hole in it . Some say that SAR and MAK barrels were turned down but I've yet to see 1, they just werent threaded (that I've seen).
Knock out the forward most pin on your FSB with a straight punch (NOT A NAIL PUNCH!!, ask me how I know
). This pin can be knocked out from either side, it is not tapered contrary to what some say (like an AR pin). Thread the barrel using a thread alignment tool (TAT) and proper die. Use plenty of lube, I used transmission fluid which contains sulfur (better for cutting so I've been told). Somebody here probably has 1 you can borrow or rent (I used to have 1 that I'd let you borrow but sold the set up). Thread the barrel till it touches the FSB, insert detent spring and pin and reinsert the FSB pin (the FSB pin is what holds the detent spring and pin in place).
Done.
Takes about 10 minutes start to finish.