Assuming an average weight of 200#, you'll need 35-40 hogs to feed 5-6000 people (figuring on 150 people per hog). If you could do a few whole hogs for show and feed the rest in shoulders, your life would be much simpler.
This might be the time to find one of those guys with a semi-sized cooker and get them in there.
Or, plan on either digging a pit or constructing one above ground, and then having lots of wood on hand to furnish coals. Also, plan on not sleeping for a couple of days.
If you do a pit, think about 4' in length per split hog, and about 5' in width, so 160' x 5'. About 3.5' deep(or the same tall if above ground), lined with solid cinder blocks on sides and bottom, rebar for support, lots of hog wire to provide a cooking grid, and tin or similar material to cover it with. The expense of this material alone is why I said that you might want to hire one of the giant cooker guys.Consistency is really everything in such a large event. Here is a picture of one I've been involved with several times over the years. It's a fire fighters fund raiser that usually feeds about 2-3000 people. In the beginning, different fire stations in the county would each bring their cookers and prepare either pork or chicken as required for that years event, but as the event grew, the cook times between the different types of cookers and cooks become so varied that the county eventually bought each station an identical cooker and sent everybody to a class so that they could produce the food in a manageable fashion. Now each station brings their cookers and everybody is on the same time table. The picture shows the result.
The good thing is you are thinking ahead and planning will be everything! My company catered the Bass Masters Classic twice in the mid-'90's for about 6000 people each time. BBQ, ribs, prime rib, shrimp, and a bunch of other stuff at about a dozen food stations. The planning and organization required is pretty straight forward, its finding the cooking and refer capacity that's difficult. Imagine 1000 slabs of ribs, 3000# of butts, 1800# of boiled shrimp, etc. It's the quantity that gets you. Christian folk or not, if you can't produce the food for this many people efficiently, you might become famous as being the Christian Woodstock of 2009! I've seen some church folks get downright primal Sunday afternoon at the salad bar.
Your food supplier will be your best source of refer trucks, but cooking will take the most planning. I mentioned wood earlier. It's a total guess because so much depends on the type of pit you construct and the ambient temp, rain, snow, etc, but I'm thinking you might need to plan on 1000# of wood per hog. (I can actually visualize the amount you'd need per hog, but the weight is a guess)
I can tell you how to do it, but if you don't have the expertise within your organization I would advise you to hire a consultant to assist in the planning and be there during the event. Seeing the site first hand and understanding the challenges is very important.