Sounds like you're going to have a fun 4th!! The advice from va_connoisseur is right on the money. I do the cinder blocks and it works fine. Just leave out a lower block so the fire gets some oxygen. I use wood and cook it down to coals in a separate pit and shovel it over as needed to the 4 corners (the thin center will cook without direct charcoal applied).
As mentioned, supply your overnight helpers with lots of coffee, chips and dip, M&M's, etc. Libations will certainly attract a crowd but be careful. Two years ago the outside of my pig was blacked due to pig watchers who toasted themselves and the pig too much. We peeled off the char and it was fine but lesson learned. Now I get some clean-cut high school age Boy Scouts to watch the pig overnight while the adults "supervise" from afar :).
I cook my pigs belly up. We used to turn it midway through but a few years back we skipped the turning and it was fine. So now we just do belly up the entire time -- much easier too. I learned that a puddle of marinade and fat forms in the stomach cavity so I poke a few small holes through it so the fat & marinade can drain off. I haven't used a drip pan as va_connoisseur suggested but sounds like a good idea. I might try that this summer to catch the liquid.
I rub the pig with kosher salt and coarse black pepper before starting it. I then use a marinade to mop on the pig every hour or so. It gives it flavor and gives the overnight pig watcher a sense of doing something more than gazing into the glowing embers all night (ZZZzzz). I recommend a base of either dry white wine or vinegar to cut through the fatty taste of the pig. Here's a recipe and assorted ideas on a marinade. Don't use a sugary tomato coating (BBQ sauce) on the pig unless you do it at the very end, otherwise it will burn and taste bitter.
Oh the timing... for that weight, it might be done before 24 hours (18-20?) but much of that depends on how well you can control the heat. Just use a meat thermometer to keep an eye on it and have some strategies to keep the hungry masses entertained in case it takes longer than anticipated. If it looks like it will be done sooner that you thought, back the fire down as soon as it looks like it will be done early. Oh, make sure the meat thermometer is plunged into the meatiest parts of the hams in several spots to make sure it is done. One spot may only read 135 degrees while other spots are done. Keep rearranging the heat so those cooler spots get cooked as well.
Ok, enough from me. Have a blast!! And we look forward to enjoying those photos you take!