I built a slant back in the late 90's that was from a '74 Valiant that had been sitting for several years in someone's back yard. When I fired the car up, it ran for a few minutes and then stopped. When I pulled the valve cover, I found several bent pushrods. Long story short: I never removed and had the gas tank flushed (I did later). Putting fresh fuel into the tank loosened the garbage and sent it to the engine. It was almost like putting sugar in the gas tank. Had the head cleaned & reassembled with a clean tank. End of problem.