Going to 6 Volts is a great idea, but it will not have much effect on the time the pack can be flown, in theory it accually will result in a little less time, the only way to increase flying time is to increase battery capacity, which in a .40 sized plane should not be a problem, I'm now using the Sanyo AA Nicd packs that are 1100mah capacity in my planes, almost double the capacity in an identical size/weight battery, of course you can really increase capacity by going with Nimh cells and loose a little weight to boot
Pete