I had a slant 6 rebuilt from a '74 Valiant that had been sitting for a long time and fired it up. Ran great for about 15 minutes then started running very badly. Pulled the valve cover and had several pushrods bent. Bottom line was bad gas sitting in the tank was not flushed out and the fresh gas simply dissolved enough to go to the engine. Rookie mistake, as I was focused on the engine and not the fuel system as a whole. I pulled the tank and sent it to the local radiator shop where they boiled it out. I pulled the cylinder head and had it cleaned. Very much like putting sugar in the gas tank.