Need advice - How do I do this?

Well, given your constraints I'd offer this novice advice.  Which of course comes from the expert advice I've gotten over the last couple years.  If by chance you have a food saver or equivalent I'd use that over zip-locks.  If not, then you don't.  4 butts is enough.  To reheat, especially if in food saver bag 30 minutes in simmering water.  Same if you believe in the integrity of your zip locks.  This good advice is thanks to @theyolksonyou and others who helped me.  It works. Good luck brother!

the ziplocks melt if they hit the pot sides while simmering, you want the foodsaver bags

For large events, I use a turkey fryer and large King Kooker aluminum pot.  If you cook a few days before the event, FS bags are a complete waste.  And for the ziplocks, I use 2 gal and fill flat.  For your scenario, 1 gal is completely fine.  And for the record, I have never had one ziplock melt from contact.  The key is to turn off the heat, and obviously try to minimize contact if possible. 

Bring the water to a boil, drop in bag, turn off heat.  Hot and ready to serve in 5 min or so.  If you load up the bottom of the ziplock bag and try to reheat, the center may not fully heat.....not good/safe.  This can happen with the FS bags, all the food is pressed into the bottom.  Another benefit is decreased cooling time after cooking, which is often overlooked.  Learned this from a catering chef on the TVWB.  One large roaster oven to heat up the first rush would be ideal, replenishing as needed.  They can be rented for cheap.  Even new, they're about $50.  If this will be an annual thing, might as well buy one.  Heating up a large amount of meat for a crowd in DOs or Crockpots is a major PITA due to lack of volume and time constraints to get to serving temp. 

You may want to up the volume to be safe, and do 4 8lbers.  Side dishes, or lack thereof, play a role too.  Men, and growin' boys(at least mine), can put it down. 
You will loose some weight in the finished product post cook.  Whatever you don't use can be kept on ice and saved for later use.  When cooking for a crowd I always err on the side of caution.  I'd rather need it and have it, than need it and not have it.  HTH 

I should have mentioned: if your goal is just having some good stuff to eat, then reheating in a some fashion is the easiest way to go. If you want to have more of a spectacle, then pulling out the cooked butts and pulling them is better and smells great.

FWIW another option would be to use a sterno catering kit.