1956 Dodge Hardtop, Stripped Oil Plug and Oil Extractor Options?

I have removed rounded off bolts by grinding or cutting a slot across the bolt head, and then using a large screwdriver with a hex shank that I can get a wrench on. Works well unless the bolt is very tight or frozen, but that's not likely on an oil pan bolt. I'd give that method a try. You can cut the slot fairly quickly with a dremel, or in a pinch, even a hacksaw blade. Make sure you go deep enough to allow the screwdriver blade to really get a purchase on the material.
Another method I've used is to weld a nut on to the rounded off bolt head, and take it off normally.