I hope you mean free floating rails? I don't think any ARs comes with free floating barrels.
Option 1-low profile gas block, Free floating rail and magpul sight on top of the free floating rails.
Option 2-Get a gas block and metal flip up sights from ARMS, GG&G, Troy, KAC, etc.
Option 3- Keep your A2 front sight. Who said you needed a railed gas block? Cheapest way to go IMO.
I wouldn't cut the barrel down anything below a 10.3-10.5in if you have a carbine gas system. If you have a midlenght, I wouldn't go below 12.5in. This is because you would need need some room for the bullet to trave down the barrel to build up gas pressure to operate the BCG.
If you go with option one, there are rail systems out there that utilizes the standard barrel nut, like the DD Omega, Troy MRF/ Alpha/ Bravo, some MI rails, Centruion Arms(my fav) which makes things easier.