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.