Large family portrait. Any tips?

[BLWNHR]

I have commited to doing a family portrait for a fairly large family. They are spread all over the world and have all come to Tassie for Christmas and this it probably the last time they'll all be in one place. They hadn't thought about it until yesterday, but want a family portrait done and it needs to be done before Christmas. All other photographers are busy, or not interested so I said I'd help them out.

I am still waiting to hear back from them on where they want to do it, but I was thinking at Royal Park (we're in Launceston) at sunset. Provided the tide is in it is quite nice. With a good dose of fill flash I think it'll work.

Body/lens will be D700/Nikkor 24-70 2.8. I have 3xSB-900's with light stands and 43" umbrellas for each. I was thinking everyone lined up in front of me, an SB-900 + shoot-through 2m above the ground on each side facing the family and me on a step ladder putting camera height about 2.5m? Would this have enough grunt to counter the sun?

Failing that I do have a black backdrop so we could do something inside, but black is hard if people are wearing suits etc.

Does anyone have any ideas/tips/samples they care to share?