Hey, saw this thread and couldn't help but reply. The method of cook you have built is referred to as a "cuban oven". They are easy, portable and can cook a pig in a short amount of time as compared to a pit. Some of the posters have great ideas and some not so much. As far as the rocks go, be careful. You need porous, igneous style rocks. Very dense rocks like granite can hold moisture and when exposed to high heat can actually "explode". I have done lots of research into different methods of pig roasting and have shyed away from the rock method due to this reason. Igneous (volcanic) rocks are hard to find up here. As for the four corners method, you are spot on. You will need to re-stoke the charcoals approximately every 45 minutes to keep the heat up. Make sure you can pull a block out from each end to add briquettes or charcoal to the corners as needed. As for the top, I use tinfoil. I lay rows down and then put the plywood on top. This helps reflect the heat back into the pit and onto the topside of the pig for more even cooking. The plywood simply keeps the foil in place during breezier weather. If you are looking for a sturdier top, I would search out old pieces of tin often found on old farm/homestead sites. An old piece of metal grainery roof or a side panel from a thrasher might work to hold heat in your pit. As for filling the blocks with sand... I wouldn't worry about that. The pit holds enough heat and the holes have dead air space that I haven't found it to be necessary yet. Maybe in super cold, dead of winter and windy weather it could be beneficial but not for any sort of spring/summer/fall climate.
I use a wet marinade and rub the pig down the night before the cook. I then cover the pig with a tarp and set in an inflatable pool then cover the top with ice to keep the pig cool overnight while the marinade works. For your set up it would be wise to split the spine but not pierce the skin on the backside so that the pig lays splayed out for more even cooking. Split from the tail bone all the way through the chin of the jaw. I lay the pig on my racks with the back facing the coals first and the cavity towards the top. This keeps the wet marinade on the pig longer during the cook process. After just a bit of halfway through the cook I flip the pig and finish the cavity side and pour some remaining marinade over the cooked back. As for the skin...good luck. Most often we have found it unedible as it gets extremely crispy and almost impossible to chew. But...I also finish a 100 lb pig in 6 hours utilizing this method so it is a faster cook. As for the meat, its extremely delicious. Not to the level of every chunk shredding with a fork, but nice firm meat chunks (main course style) and some shredded meat from the outermost cooked areas available for sandwiches.
As for portions, last year I did a 140 lb pig. An error happened at ordering so I took on the task of this enormous beast. It took close to 8 hours for the cook. I fed over 70 people (some children included) and easily had enough pig left over to feed 75-100 more folks (also considering side dishes). This year I had a 90lber and fed around 60 folks with lots of pig left. Lots.
Also, don't be afraid to throw some chunks of mesquite, hickory or apple into the briquette piles. It will help with the smoke flavor. And don't get rid of your moisture!! Moisture is your friend!! Just don't allow the condensation to drip onto your briquette piles. Make sure you insert the thermometer into the deepest part of the ham and shoulder joints, where the ball/knuckle bones are. These are the last areas to be finished undoubtedly. USDA now says that 145F is done for pork but most people would balk at putting that pink colored pork into their mouth so I usually take mine to 155F. I try not to exceed 160F on the inner portions. At 155F, the meat is borderline starting to get firm (but not dry) and has mostly lost that pink tint to it.
As for your grates and your pit...looks spectacular. Not sure you need that air vent, the pits I build every year I line with foil at the bottom and I still get plenty of ventilation to stoke the coals. Also, your grates are much lower than where I put mine and you have more wind block at the top with those extra courses of block so that could increase heat exposure to your pig so be careful you don't overcook. Just a thought but you might actually have to add charcoal/briquettes at a less frequent rate due to your lower grate design.
Good luck and let us know how the swine turns out. Pictures preferred!