The problem with giving too much aspirin is it also thins the blood. It might cause the stiches to leak.
Hottie weighed 100 pounds at the time the vet had him on aspirin. He got one and half 325 mg asprin twice a day. For a 30-40 pound dog, I would say a half of a 325 mg asprin would be safe. But I would check with the vet to make sure they do not feel it will thin the blood too much.