One petcock leaves only 4 oz.in the blanked off side of the tank. That's why Yamaha went with single petcocks in late Specials. The center-to-center distance between mounting holes from 1978 forward is 46 mm. A simple piece of steel or aluminum bar stock and a hand cut gasket will get the job done if you want to run a single petcock. I'd suggest the manual replacement from Mike'sXS, but I don't know if the same quality has been maintained over the years. All I can say is they used to be, and maybe still are, excellent. Anyway, the blanking plate doesn't have to be thick: 3/16"will do it. Use Teflon tape and copper washers on the screws and a good sealant like Gaskacinch on the gasket and you'll be good to go with a direct replacement for your current tank. It really is that simple.