Tom is right on his numbers. He jokes about not being a pro, but he's pro to the core.
Sides are hard to figure because people don't eat like they used to. I had a party this weekend that only ate about 20% of the beans I took, but ate slaw like they were cabbage farmers. Followed by a party the next day that had gone through all their beans with people still in line, but barely touched the slaw. They however hit the pasta salad pretty hard. Go figure.
As far as spiral pasta, first of all, Sam's sells this in 6# bags for about $5. 6# makes about 16 quarts, and I usually figure 6-8 servings per quart, so each bag will make 96-128 servings. I'd use the smaller serving size for your group, so three bags should do you. Your mother in law can also get the pasta in 10# boxes.
Sam's also has a pretty good Italian dressing in gallons for $7. Toss the pasta with some diced cukes(green) or broccoli, diced red onions(purple), diced red peppers(red) or grape tomatoes, sliced black olives, and some crumbled feta cheese and you have the makings of a cheap and colorful pasta salad. The Italian dressing is a little safer than a mayo based dressing.
DO NOT buy 500 hamburger buns unless you want to fatten up the birds. It is very unusual to average even one bun per person unless you're making the sandwich for them. For 320ppl, I'd go with ~250ish and still plan to have some left over.
If this is to be self serve, I'd go with 9" plates. This will cut down on folks that pile on the food then throw most of it away, but still allow for plenty of food. Then they can come back if they want to get more.
Do as Tom suggests and do a cheese and cracker tray with pickles and smoked sausage. This will serve the dual purpose of giving you some wiggle room as far as time is concerned, and also some wiggle room as to portion size. Do a BBQ dipping sauce, a peach(or pineapple) mustard sauce, and then some pretty hot straight mustard, maybe chipolte or chinese mustard. Everyone will be happy.
EDIT: Also, you might want to cook some chuck roll instead of brisket for the chopped portion of the beef. You'll get better yield since the roll doesn't have as much fat.