Ok so i have to do a wedding shoot tomorrow (my first one) and it was calling for blue skys which was nice but now they have changed it to overcast with possibility of rain... so the rain isnt my worries because then the photos will be moved to another day aswell indoor

what i am looking for is hints or tips to make an overcast day look better
i am using
-Nikon D70s w/ 18-70DX lens
-Nikon 5700 (backup)
-Nikon Speedlight flash i think the 600 (im trying to borrow it if not some ancient canon flash)
*and i have made a diffuser for the flash