Flashing with ambient light.
What your up to is called dragging the shutter. Once you have the main flash worked out, you drag the shutter with a longer shutter speed for the background exposure. This is a pretty basic studio thing to do, depending. It's also possible to second curtain the flash synch with subjects in very dark conditions. A low ISO film would be prudent here as it will build foreground exposure slowly causing less subject movement exposure. Most standard photo books cover this.
To understand what you want to do, figure a reflected meter reading will give you a zone 5 exposure for the background. If your aperture is set at say F8 and at 15ft for the flash exposure according to a guide number, you'll find the amount of difference between foreground and background in the shutter speed settings when using a manual flash; Think studio flash here as it's easier to understand. A ttl flash model will just increase the flash output compared to a nuts and bolts, this is what your getting out of me manual flash. Hypothetically, if the flash synch for this picture is 1/60 at F8 according to your meter for the group exposure and the medium zone 5 back ground exposure is 1/15 at F8 , your going to be 2 stops down in exposure on the background at 1/60th. So where does that put you with your film latitude and what your trying to record? Probably pretty good if you have brightspots in the sky. Sometimes as I mentioned above what you want to do is second curtain the flash, expose for the background placing it where you need it and then flashing the main subjects in TTL. The best way to figure it all out is to test. Since it's a wedding, a half black, and half detailed white test board would be great to use. If you can stick it in the ground use a 5ft tall stake and a peice of painted cardboard. Try using NPZ. For all accounts most rate it at a 1/3 to a 1/2 stop down in speed. Say ISO 640 or even 520, and it will boost contrast a little too.