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The only suggestion I might add would be to gel your flashes with 1/2 cut of CTO or so. You'll be trying to make a reasonable blend of mixed lighting, from the ambient early evening to cars' headlights and then your flash (which will be much cooler). With those light sources, if you don't pay careful attention to color balance from your flash, you're probably going to have some issues. I don't think you want cold flash light in an otherwise warm environment. If I had this assignment, I'd do some practice shoots using this specific combination of lighting sources before the actual shoot with clients.
Having said that, from what you've described, a somewhat gritty black and white conversion might also be a good look.