The confining-restraining action of surrounding concrete seems to be quite effective, what you can see in the factors of conversion for strength between test cubes and cylinders. A cube of just 20 cm sides shows significant higher compression strength than 15x30 cm cylinders. So I wouldn't be very concerned as long one has a complete path of the loads.
A foundation ratio of L/B greater than 10 can accurately be considered a infinite strip for a plane strain condition. Greater than 2 can be modeled with plane strain condition depending on constraints and the model you are using. A L/B ratio of less than 2 should generally not be considered an infinite strip when considering plane strain, however other factors such as the dimensions of the footing and soil can come into play.
