Maybe you could weld in a plate just around the sump. If that's all you want to protect, why use a big plate that's more easily torn off? And either leave an access hole in the small plate for draining oil, or leave it as solid plate and pump out the crankcase with one of those electric pumps.
How do you feel about raising the car a couple inches for ground clearance? Not optimum, I know, but better than a hole in the oil pan.