You have a standard FSB? And want to keep it? Do you have carbine length gas with standard 7" handguards? If so 7" two-piece drop on rails should work but be careful that SIG truly is standard length and uses a standard delta ring and end cap mount.
If you want to replace the FSB there is more to deal with.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek