Willie, thanks for the vote of confidence, but I suggest starting a new thread on the Barbecuing forum titled something like "Four 10lb butt suggestions for the 18.5?" You'll get a lot more help that way, especially from the few that have cooked that much.
Basically though, if your cooker has good fit and you've had no trouble managing temps, I'd suggest using a clay pot base instead of water in the pan. (I think a 14" would fit in your Weber pan, but I use a Brinkman pan with a 12" one.) If you're nervous about that, only start off with one gallon of hot tap, and then add more after the cooker gets up to temp.
I measure with a probe in the vent and try to keep 235-250* after initial heat up. If you go by the Weber dome therm, you'll be able to relax some once it reads 200*, and I'd be fine as long as it doesn't go over 250* since you've got a full bottom rack. As for temps and times, I'd probably allow for at least an 18hr cook, maybe one or two more depending on the weather; flipping/turning the butts 180* 6 hours or so into the cook and hopefully they will have shrinked some so that you can make sure there is some space between them. (I'm sure others might know better since I've had no more than two nine pounders or so on a grate and can always make a little space between 'em.) The cooker will be slow to come up in temp, but that's ok. Don't be afraid to leave the vents open for a long time. I've done overnight cooks a couple of times where they STAYED 100% open til the next morning, but like I said, my cooker is pretty tight and cooks low.
I wouldn't let the meat sit out except for maybe half an hour, so it takes the rub easier. This all depends on your schedule, and it's perfectly ok to pull butts out of fridge and out of bags, let sit on counter while you mix a rub up, rub, then throw on the cooker.
I use a torch and usually water in the pan on a rather slow cooker, but if going dry on my cooker, I'd probably start with 1/2 a chimney. Every cooker is different, but my basic idea is to try not to have 275*+ heat hitting the bottom rack outside the pan, and this can be checked by use of a fairly accurate cheap oven therm placed on the bottom grate right inside the door. If you're careful, you can rotate the cooker on the base and check all sides, but don't worry about it too much. Pork butt is pretty forgiving. More people undercook than overcook, and if you're cooker has been on the "slow side", you might want to allow for a full 2hrs/lb since they'll be no space between the two on each rack.
Expect your bottom rack butts to be done first since the rack will be so full, and that's what the door is for. (Position the butts so you can check both through the door.) Personally, I'd rather cook til tender where you can turn a fork in 'em and have no guesswork on the residual cooking. In other words, after they're done, tent with foil a while after cooking and then hold in a cooler for hopefully no more than two or three hours. You just have to use your judgement when deciding how hot to hold and when to cool and reheat if a cook gets done way too early.
