How high is the overhead and what color is the underside of it? If it is white and your flash is capable of bouncing, then as much as possible bounce the flash off of it to avoid harsh, unnatural, flat lighting that comes from straight-on hotshoe flash.
By way of example, here are a couple of casual shots I did recently by ceiling bouncing a flash (AF280T) in auto mode:
Pentax K20D
Cosina 19-35/3.54.5
Pentax AF280T
Bouncing the flash really makes a world of difference compared to straight-on.
If you can have access to the area then grab somebody to go with you over there and practice practice practice shots beforehand at the actual location.