The Naish All Terrain is more of a bump & jump sail. It will work, but not ideal as the Futura is more of a slalom board. The Naish Sprint or its predecessor the Naish Rally would be a better match for the Futura. Even better would be the 2 cam Indy.
42/40 should be a fine match for a 6.5 at your weight. I weigh just under 80 kg and use a 38 cm Tectonics Talon with a 6.6 Sprint on my Futura 133.