I've cooked quite a few whole pigs ranging from 60 to 125 pounds. The bigger ones are difficult to handle once they're cooked.
The method I use for the pit is described here
3 Guys from Miami roast a pig! How we build hog roasters.
I don't usually butterfly the pig, preferring to have it laying upright.
It looks better for the 'viewing'. Also, better if someone gets challenged to kiss the pig.
The slow indirect heat with a good marinade gives a taste that is hard to beat.
I use a water trough with ice water in it to keep the pig cool during the marinating. The pig and marinade goes in a couple of large plastic bags. For a 80 pound pig I'll make up about 4 gallons of marinade and let him soak for at least 12 hours, turning him over periodically. Prior to cooking I'll stuff garlic cloves into the hams and shoulders and do some injection as well.
If you want to present him with an apple in his mouth when he's cooked, use a baseball during cooking and switch to the apple when complete.
cooking time is dependent on the pig and temperature. With the cinder block pit it's fairly easy to control the temperature once you've got some experience. Just don't expect to have the pig done at 6:00 sharp, relax and enjoy.
However, if folks are going to just show up, eat and leave - throw some pork buts in the oven. This is only worth it if the crowd will be there for a few hours to enjoy the smell and company.