I tend to get around a 55% yield on pulled pork.
For 75 people with 4 Oz servings 2 sammies / person, my figures say 68 lb uncooked to get 37.5 lbs cooked meat with no spare, (or not much anyway), so 10 x 8 lb you will have extra there.
I would go up with the serving and go 3 / lb or 5.33 Oz / portion, you can also say, "If you're still hungry come back for a second!" You may get 50% takers but I bet you don't.
75 @ 5.33 Oz is only 25 lbs cooked, 45.45 lbs uncooked or 5.5 8 lb butts, so if you went with 7 butts and 5.33 Oz you would still have spare.
The main reason I suggest going up to 5.33 Oz / portion is, you will get less wastage for the most part, a larger number of your clients will be satisfied with the bigger sammie, and those who know they can eat a second will do so, if you give everyone 8 Oz to start out with, those who will be happy with a 5.33 Oz would eat about as much, but you are wasting the other 2 - 3 Oz on each plate that doesn't get eaten.
You are about right on your beans, but they are cheap, so bumping it an extra can won't break the bank either.
"There's no such thing as a little garlic" A. Baer
KCBS certified barbecue judge.