Since the oil pan is more or less at atmospheric pressure (i.e. non-pressurized), you can easily make one from gasket paper or use RTV sealer such as Permatex Gasket Maker. Not factory or concours correct, but probably better than original.
I just made a pan gasket for my cast iron 2000. Local auto parts stores rarely have the size and thickness of gasket paper for all applications. Fortunately, I have a Summit Racing warehouse not far away, and was able to buy a roll that was both wide and long enough, as well as fairly thick. The 2000 originally used a soft cork gasket to accommodate the likely variability in the pressed-steel oil pan. I don't know what the 2600 uses, but if it is the wobbly pressed-steel type, this is a good way to go. If it is a cast aluminum pan, then a thinner paper is likely to be fine. These can usually be found at local stores.
I recommend buying either a hammer-style punch with various diameter tips, or a universal punch such as can be found at Horrible Freight. Both make nice bolt holes compared to carving them out with an Xacto knife.
Took me about 15 minutes. Easier than chasing down an original gasket that may be old and slightly shrunken.