not enough info to say for sure, but if you can get that close, an axe would eliminate all guess work. Suprisingly enough an opposum is very strudy. Dense muscle structure, dense coat, and thick skin, similar to a cat.
As a test, get a bar of Ivory soap, and an empty drink can (aluminum) fill the can with water, set can up at a safe place to shoot. Place the bar of Ivory soap about one inch in front of the can. Shoot through the bar of soap and into the can of water. If you can shoot through BOTH with the same shot (and I mean a complete shoot through ) from the distance you will shoot the opposum then if you aim at the nape of the neck, or through the ear canal, you will have a chance of ridding your yard of the invader.
without chronograph data and exact bullet weights this is my best guess
Keith