6 hours with a full load shouldn't be too much of a stretch for the 800 W.
Takeoff weight is 79 tonnes and landing weight a touch over 66 tonnes - max zfw is a touch over 62 tonnes.
That means that you can load the aeroplane up to max ZFW, stick nearly full tanks and launch off into the wild blue yonder.
The difference between max takeoff weight and landing weight is about 6 hours - the only issue would be island hold requirements, which you could probably deal with if you put some thought into it..
the 800 with winglets is an incredibly versatile aeroplane and the 700 with winglets is as good.