I've got a custom 18" barrel, rifle-length gas system, carbine-style buffer/stock, and FA carrier. It runs fine with a standard weight carbine buffer, but shows signs of being a little over-gassed. (bounces brass off the deflector) Your mileage may vary, but I've had better luck with an H2 buffer in it.
Assuming your barrel uses a standard port size (like all of mine), the longer barrel could (theoretically) lead to even more over-gassing from the longer dwell time. So, that in mind, I'd suggest starting with an H2 buffer. That will get you very close in weight (within an ounce as I recall) to a standard rifle buffer without going over. It's possible you could still be a little over-gassed, but I suspect that will leave your reciprocating mass low enough (even with a FA carrier) to cycle reliably with any standard .223 or 5.56 load. Then, if you find it still over-gassed, you can always go to an H3 later.
Good luck!