I too am in a similar situation. However, I have worked as a second shooter for another wedding so I'm fairly confident in my ability to get shots at the reception. The scariest part for me is the bride walking down the isle. Just one of those shots you can't, can't, can't screw up.
My idea (and please offer advice, feel free to tell me i'm wrong, maybe even stupid.. lol) was to stop down some f4.0 at least, high shutter speed 1/160 at least, and use iso, flash for fill light. I just want to make sure focus sharp, everything is properly in focus, and no motion blur.
FWIW I'll be shooting 5dmk3/7d, 600ex-rt/430ex2, 70-200f2.8, 50 f1.4, 16-35f2.8