We need more information to be able to provide guidance.

First, how high are the ceilings?
Second, are there risers (or stair steps) available?
Third, what will the photos be used for?
Fourth, what lenses do you have available for use?
Fifth, what other lighting sources are available (windows, etc.)?

I can tell you that, as a general rule, 35mm doesn't cut it for large groups
(film or digital).  Neither will a single flash.  But, give us some details
and we'll try to help.

Walt Hanks

I know a lot wedding photogs would go ahead and blast away with a 35mm and a
flash, but there is a big difference.   A wedding has lots and lots of
pictures of the main stars and immediate family, a large group is just a
cattle call of everybody in the bride's or grooms extended family, you just
add more and more people in there, the couple will probably buy it anyway.

A group portrait requires that each and every person get equal time, well at
least equal space and light.  That guy, (or gal) way up in last row on the
right (or left) just might end up being president or indicted or both.

So even with modern film, or high res digital, I don't think its going to
cut it.  Even back when medium format was the definition of professional,
the advice would be to get the big gun out, the 4x5.  well come to think of
it, WAY back when photographers used 4x5's and those rollies and blads were
lumped in with the other miniature cameras they would say, well a big group
like that requires more detail yadda yadda, and if you look it up, there are
or were cameras specific for this kind of shot, the banquet camera and IIRC
were 11x17 or 12x18 something like that.

but still, how many pixels?  2,000 across the horizontal, 40 people, about 4
rows of ten, with space between heads you're looking at 60-70 pixels per
face.  maybe?

maybe if you had one of those 11mp cameras.

so even today I would consider renting a medium format camera.

now remember, each person needs equal time/space etc.  I would seriously
consider using a slightly long lens, a 110mm would be ideal, its still a
portrait, a wide angle lens will get them all in, but at a cost to the folks
in the back rows, and especially the ones on the ends and you don't want the
next president or psycho-killer whom the media will be sure to pull out of
that group shot looking like their face was run over by a steamroller.

rent a power pack and a couple heads or a couple monohead flashes, put them
up high on either side of the camera, NOT out far so you get separate
highlights, but close so they just give you a wide flat light, bounced off
brollies.   You can use your flash for a fill one stop under.

This reply is echoed to the z-prophoto mailing list at yahoogroups.com

The Metz flash you mentioned will give you plenty of light, but 35mm
is stretching it a bit.  I wish you could borrow or rent a MF camera.
If you cannot, set the camera to f4 and the flash to f8 and make SURE
you can see everyone's face.  Use 400 or higher film and a 50 or 35
(1st choice) or 28mm lens.  Don't shoot to the edge of the film, leave
a little room. (Risky with 35mm but different sized prints crop
differently.  You don't want to cut off the folks on the end on a 5x7
when they are visible on an 8x10.)

Pray a lot, and as long as you don't go beyond 8x10 they should be on
the edge of acceptability.  If you have to work with what you've got
then that's all you can do.  What you are hoping for is that MOST of
them will be pleased.  And you're not going to please ALL of them
whatever you do.  With this same setup I have taken shots that
produced beautiful 8x10's and shots that made lousy 8x10's.  If you do
all you can do to prepare, the rest depends on the room.  

Also, I've never had any luck trying to take more than one shot and
piecing them together.  It's a lot of work.  If you've done it before,
fine.  But a big job like this is not the time to experiment.

Good luck!

BBFoto

Here are a few more things to think about.

Without some sort of stairs or risers, about all you can hope for is
three rows of people.  Conservatively, that means a 30 person group
is going to make for a 20 foot wide shot, and probably more.  Since
your flash only covers a 35mm angle at full power, you'll end up at
least 20 feet away from the front row, shooting at f/5.6 or so with
160 speed film.

The DSLR's crop factor puts you at 32 feet, at which point you
may need to drop to f/4.

_Somebody_ is going to blink or sneeze on every single shot, yet
trying to hold that size group together for more than half a dozen
tries is not going to be easy.

Unlike other posters, I wouldn't worry about unflattering results due
to the use of a wide angle.  That's not directly a function of focal
length, but of distance.

Having people stand on chairs, at least in the U.S., is an invitation
to a lawsuit.

The good news is that you don't have to worry about too little depth
of field.  The bad news is that the reason you don't have to worry is
that you've only got about 6 or 7 square millimeters of film per face,
and even less sensor space on a DSLR.

You need to get the flash above the second row of "heads," otherwise
it will cast shadows onto the faces of the people in back.  Even at
best, the lighting is going to be fairly harsh.  You probably don't
have enough flash to bounce.

In short, my advice is to hire a pro with the equipment and experience
to take this sort of large group shot.