Free floating will still make a difference on a chrome lined barrel, especially if you use a sling mounted to the handguard for shooting offhand, or if you mount a bipod.
Honestly, you should have ordered a complete upper. Putting one together and getting the gas system lined up, and properly securing the gas block (assuming you don't have matched holes in barrel and gas block for taper pins) can be a real pain in the ass.
As for the earlier question of staking the castle nut, I have never had it done on a single AR, and I have never had one come loose, even when run hard. I have one guy build all of my ARs for me, and he has built far more rifles than anyone on this board has ever owned, and he doesn't stake them. I get the feeling it becomes less of an issue when they are properly tightened.