+1 on the bouncing, but the calculator on the flash won't do you much good, as you will lose at least a couple of stops, I would think.
If you can meter the flash with your Sekonic that would be best, or else tilt the flash head and try auto mode on the flash. If you can keep the people together long enough, I'd use every method presented in here.

No matter what, your flash is going o have a hard time lighting that room on it's own, and if the interior is dark wood you probably need to forget about bouncing and use the flash direct.

Hate to say it, but, you may also want to consider multiple shots the could possibly be stitched together using that "numbers" method.
Because, depending on what the group and the room are like, you'll be hard pressed to get good light everywhere, especially since the Metz's rectangular head doesn't match your Hasselblad's format very well, though with the 80 that problem will be minimized.