There is also a check valve that Northern Tool sells that will automatically stop the gravity fuel flow when the fuel is up to the filler neck. I have this valve in my setup and it never overflows/overfills. Pretty simple device, sells for $70. I have used this valve on 2 different trucks and it has never overflowed. Item # 3490901 or 02, depending on brand of truck.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired